Negative pregnancy tests

I had a baby in June and have stopped breastfeeding and have had two periods since. I haven't gotten my period this month, but keep getting negative pregnancy tests. I bought the tests on Amazon in bulk. I was wondering if maybe bc I stored them in the hot steamy bathroom they are no good. I have an appt with my obgyn for my annual tomorrow. I will be asking for an ultrasound or blood work. But was wondering if anyone had similar experience and ended up to actually be pregnant? :) Thanks!