PCOS? Pregnancy? Menopausal?

I'm a 28-year-old, relatively healthy (aside from being slightly overweight) female, and I've always had long cycles (avg 35-38 days), Very rarely I'll even skip a month, but lately everything has been all out of whack.  My last real period (7 days) was in November.  I had two days of spotting in January, and about a day or so of spotting in February.
I've already taken two pregnancy tests (one in January, one in mid-February, and one last night), and all three came out negative.
Does anyone know of anything (aside from pregnancy) that could cause two to three months of skipping your cycle?  Polycystic ovarian syndrome (PCOS), maybe...? I don't have most of the symptoms, though.  Just the long cycle, which I guess could be considered irregular....?  I'm a little overweight, but I'm pretty sure it's from eating junk food and not exercising (which I'm currently working on fixing 😊).
Is it possible to test negative and actually be pregnant?  Surely if I was, then my body would be producing SOME HCG, and I've used both the First Response test that gives you an early result (so it's extra sensitive to the presence of HCG) and the Clear Blue digital test that's supposed to tell you how far along you are (if you are).  Both clearly said not pregnant.
Starting to get a little concerned, and I'm wondering if:
A) this has ever happened to anyone else 
B) anyone has ever heard of a woman being pregnant and receiving a false negative multiple times on a pee stick
C) anyone has any idea what (if anything) could be wrong medically-speaking, or 
D) all of this is normal and I should stop stressing out over it.