Can I be pregnant ...

I had a baby almost 4 months ago. I did everything I was supposed to do to not get pregnant... I took several pregnancy tests but it looks like it’s the most faint line I ever seen.. idk.. and the last 2 days I all of a sudden feel nauseous & everything sounds gross to me (I hope I ate something bad) should I go in and see a dr for blood work? On my period app it says I’m 13 days late. The red means I was on my period. The hearts mean sex of course.