Hi p1994, I'm sorry you've been through/are going through such a hard time. But 2 months ago really isn't that long ago and you may need time to adjust to whatever resources the hospital gave you, and more support in moving on from the problems you've had/are having.
Are you seeing a therapist or pdoc regularly following on from your hospital stay? If you are then I'd say you really should let them know about what's going on for you as soon as possible. That won't necessarily mean that you have to go back to hospital, but it should get you the support you need to move forward. If you're not seeing anyone, then I'd suggest you contact the/a hospital to have them see you in outpatients to talk to them about what you're feeling.
As for the "why"'s there could be plenty of reasons e.g. not enough coping resources, depression, anxiety, your situation at home or elsewhere, things that may have happened for you, addiction to the SI................or even all of the above!! But whatever they are you really need to talk to someone/a professional about this. You've clearly come so far!!! Don't deny yourself the chance to move on more and past this, you're worth it!!
Alison