Late but negative?

We started ttc #3 in October, no luck so far. I had a positive opk this cycle but i wasn't hopeful because we dtd once in the window. My pms acts like pregnancy but this one the nausea has been insane and I've had to pee a lot (no fever or indication of infection). af was due yesterday and still hasn't showed. I took a test one day early and again today (one day late)both negative :( they're not the first response because those are expensive and it took 4 years ttc for my last child. Is it possible I'm preggo and just haven't built up enough hcg yet?