Positive then negative tests???

So my son is almost 10 weeks old. I got negative tests after birth (on multiple separate occasions as the midwife made me do one a day for the first two week PP as they thought I might have retained some products.

Anyways we started having sex again at 3 weeks PP. I took a test on Monday that was positive. Then I took one yesterday that was negative. Now I don't know what to think🤔🤔 I'm going to test again tonight but what are we thinking? Maybe the 2nd test wasn't as sensitive? It was a 2 pack and the other one in the pack was invalid...

I don't know what to think!! I hope I am pregnant (even though we did uses condoms as we weren't trying but they split on 3 occasions during the last 7 weeks) I will test again but my partner is now convinced I'm not pregnant and says it's a waste of time but I know he'd be upset if I tested again without him