If you had unprotected intercourse
in December and had periods as usual (of course with minor variations you have mentioned), the chances of pregnancy
are least likely.
Some times excessive concern about the possibility of pregnancy and the stress
can cause hormonal
imbalance and may result in variations in your cycle you have mentioned.
Still if you want to be 100% sure, you can go for the following tests
1. Serum Beta - HCG (It's hormone increased in your blood if you are pregnant)
2. ultra sonogram
But i don't think they are necessary in your case..