Period is two weeks late, bad stomach and sometimes feel sick? 16 years old.

Hey, Im 16 years old nearly 17 and I've been sexually active for a few months now. I've always used protection, and also planned to choose another form of contraception (pill etc), and they told me to come back when im next on my period.

My due date for my period is generally in the middle of each month; sometimes the gaps between each are shorter.

this month I havent had my period, and I'm two weeks late, which has never happened before. If i think back, im not 100% sure of when my last one was either. I've been having bad stomach aches and feeling sick, and if im not imagining it; i think my stomach is slightly swolen too.

I've taken pregnancy tests and they say negative so I'm wondering if its anything else? Im quite worried.

I've had the injection contraception before and I think it affected me quite badly. i had bad mood swings and i felt reoccuringly sick whilst on it. It ran out Febuary 17.

I plan to go to the doctors if things dont sort themselves out, but im just hoping to get a general idea of what might be going on

Please reply, thankyou x