normally you can not get pregnant while on your period... NOT to say that it hasnt happened... just that it is rare. it would mean that you were ovulating AND shedding lining at the same time. very rare as you have to have a lining for the egg to implant after conception.
that being said... for MOST people ovulation generally occurs 12-16 days BEFORE your next period. with the average norm being 14 days. and there are only a few days before/during/after that day that you can hope to conceive IF you have sex at that time.
i would take a test, and then start charting your cycle to find out exactly how many days your cycle is and when you ovulate. that way you have NO doubt as to when you ovulate or when to expect your period. i would also get on some form of birth control as you are very young to be getting pregnant.
good luck and hope you get some answers.. all you can really get here are opinions cause the only one who can give you answers is a doctor or hpt... which i think will be negative.
are there clots in the blood? (i know gross but that's one way to know if it's a normal period or not) when i was 17 and even now.....my periods can range from 1-3 days early to 1-3 days late. you can take an hpt and you'll know for sure if you're pregnant. and like martika said for your peace of mind b/c and condoms would probably be a good idea.
Also, this is NOT a negative comment and I'm sure you know. but in the future if you want to prevent pregnancy get on birth control or use a condom. :)
Firstly, I want to quickly comment on your statement that you always get our period on the 20th or 21st of each month. This very fact shows that your cycle ranges from 27-32 days. The timing between your last period and this one falls within your "normal" range so I wouldnt be concerned and consider that atypical for you.
That being said, you can get pregnant at any time during your cycle. A home pregnancy test will usually give a positive result about 2 weeks after conception (a little earlier depending on certain conditions). I would recommend you take a HPT now (should be accurate if you conceived during the last week of March) and you can retest in a week if its negative and you are still concerned.
My personal opinion is that you are not pregnant but an opinion isnt an answer - only a test will give you that.
Good luck.
Sounds like you are on your period. The only way you'll know for sure is to test, but to me that sounds like you are just having an earlier period. Sometimes it gets delayed or it comes early, that's just nature...
Please use some sort of protection, hopefully condomwith a spermicide when you have sex to avoid any unwanted pregnancy at an early age.