Have you been having unprotected sex all throughout, such that you might have gotten pregnant between the test at the doctor's and now? Did the doctor use a urine test or a blood test? Have you taken home tests in the weeks since you were at the doctor's?
So, what IS tricky when we don't get a regular period is knowing when we might ovulate. This is how unexpected pregnancy can happen. Double edged sword as it is also what would make someone with very irregular and long cycles have a more difficult time conceiving. Because you can't predict anything.
However, If you haven't had unprotected sex since September, you'd have gotten a pregnancy test most likely 2.5 weeks after that. There is a very rare person that doesn't set off pregnancy tests. It doesn't happen often but it does happen once in a blue moon. I wouldn't think this is you.
but you should see your doctor. They should, at this point, do blood work on you. I had cycles every 3 to 4 months (period only that often) in my 20's for a year and a half due to a combination of hormonal imbalance (naturally made too much estrogen) and hypothyroidism (slow functioning thyroid). I had PMS symptoms though starting at 3 weeks after a period and I'd have them the entire time until I got that really infrequent period. PMS symptoms can feel like pregnancy symptoms. It can be confusing when we don't get a period.
Blood work from your doctor would be very telling.
OKAY, see at the end you did this. Sorry! Did they also check your hormone levels? This would be important. And what did they propose to get you going again? Some women will take the pill to get a period again and kick start their system. What is the doctor doing for your 'next step'? If they didn't set up a next step, I'd go back and ask for one.