Stay at home mum blues

I'm a stay at home mum and sometimes I find myself pacing the house because idk what to do....my baby is almost 2 months and I have a 4 year old. I should also mention I have a chronic pain condition so sometimes I just need to rest and I can't because I am a mum. And then I feel guilty like I should do more with them or something. When I was pregnant I was always doing stuff with my 4 year old and now I just feel so blah I don't. I also don't have a vehicle so I can't bring them places & because of my condition the thought of a walk seems unmanageable. Idk if I have post partum or just need a me day...idk if I'm going stir crazy being in this small apartment with them 24/7...the only adult contact I really have is with my bf and by the time he gets home from work he is exhausted and just wants to sleep...I just want to be happy....I feel like I'm just blah all the time..unless I have a lot of tasks to complete for the day...I don't even have the energy half the time to do something indoors with my 4 year old (again my health condition) I'm just starting to feel like a horrible mum...any input any advice?