Hey, im no expert on this but it could be worth speaking to your partner or a trusted friend, family member and asking what they think as they will notice changes in behaviour you maybe aren't aware of?
I find its so much better to talk about these worries because the more you keep it all in the more these thoughts just fester (thats really for myself FYI haha).
Are you active? As in getting outside every day? I personally find i feel so much better getting out. Also engaging with likeminded women helps (for me NCT people, haven't yet tried any baby classes)...
I have a 1 month old and find some days are amazing, others are the lowest. For example a few days ago i was just insanely sleep deprived, i felt like i was going to collapse i was so tired and that affected my whole ability to think straight.
I couldn't stop crying for about an hour, and actually called my husband at work in tears saying it was all "too hard". My sister was around to help with baby but on that day it all just got a bit much.
Up to then i was surviving on broken sleep of maybe 45 mins at a time, total 5 hours per night for 3.5 weeks. I was burntout (as all parents are)but to the point i had to do something different... for past few days I've expressed around 180ml total for 2 x night feeds at 9 and 12 and my husband feeds baby so i can sleep 8.30 ish until next feed at around 2... LIFESAVER!
Ive had to make it a priority to get more sleep and take care of that basic need as if i don't get block sleep i just can't function well long term for baby. Maybe this has something to do with your mood?
Also, there is so much support if you do have PND and if you are really struggling would def recommend seeing your GP to see what could help. Just remember to make yourself a priority too as your wellbeing is just as important as babys (easier said than done i know!)