I don't like going outside anymore

There could be a few reasons that's happening.  If you are afraid of leaving your home and going other places, that may be an anxiety response.  Not all people who feel this way have all the criteria for agoraphobia, but that doesn't mean it's not hard and doesn't interfere with your life.  This may be triggered by a fear of being around people or people seeing you.  It may also feel like leaving the safety of your home is too overbearing without a clear indication as to why.  When it is the latter, it's typically brought on by an anxiety disorder (such as panic disorder).

If you have lost interest in going outside, that may be a very different clinical picture.  Did you used to enjoy getting outside but now feel like it is no longer enjoyable?  Have you lost interest in other things that you used to find joy in?  When depression is the culprit, it can feel like you don't get excited about things you used to like, which is also accompanied at times by feelings of worthlessness, hopelessness, sleep and eating changes, and frequent sadness.  

It's not uncommon for anxiety and depression symptoms to coexist, so you may be experiencing a little (or maybe a lot) of both.  If you feel like you need to speak to somebody about what you are going through, counselors can usually help and with the option of telehealth (online counseling) you don't have to leave your home to do so.

If you don't have access to a counselor, your primary doctor can also help you understand your symptoms better and maybe even recommend some other options for you.  

Social anxiety has skyrocketed since COVID.  Plus could be trauma or PTSD.  Maybe you are a known person and feel safer not going so much out in public.  I understand as a known person in my field depending on where I am it can be a bit much.  Or maybe the things you found Joy in before are not as much of an interest.  It could be your age or menapause (hormones) and injury.  SO many things could be in play depending on the client and their lifestyle.  Always best to get a check up.  See if there is under lining medical issues and then go from there.  

One reason may have to do with physical anxiety symptoms. This is common for people who experience panic attacks or physical anxiety symptoms. Often it begins with just avoiding certain places or activities where the symptoms happen. Over time, it's common for it to spread to more and more places and situations until many people get in the habit of not leaving their home at all in an effort to avoid the symptoms. 

The short answer I could give is that we don't know why it's happening. The longer answer could come from deeper exploration as to your fears, or anxieties, that you experience maybe just thinking of going outside. But any sort of exploration about this almost HAS to begin with an agreement within you (and all of your feelings), that's it's not something to be ashamed of. If we are ashamed of any of our behaviors, then we lose the ability to sit with them and see where they are coming from. 

This could be something that can be worked through in therapy, and specifically with a therapist who specializes in anxiety and more specifically, agoraphobia.

The first step to answer your question is to reflect on what is going on in your life overall.  

If you're in the midst of severe stress, whether within your relationship or effort to establish a relationship, upheaval in family dynamics, some type of significant loss like a job or someone with whom you feel closely attached, then you may simply be creating the solitude which is necessary to better understand and absorb the meaning of these changes.

I assume you mean you are by yourself when you are indoors.

What you write sounds like you're not happy with this recent urge rather than that you and someone or others are having great and fun times together.

One way to encourage yourself to go back outdoors is to connect w a friend or search for an interest within you to develop.

This way with either of these, you'd feel a little bit of motivation to join and get more involved with either of these.