Could i be pregnant or could it be some other issue? 21 years old with no medical?

ok my period is normally right on track every month but since november of 2010 it has ether been skipping months or being very light ether short or for two weeks and i also noticed i had gained 50 pounds very quickly and my friends at the college have been asking me everytime they see me if i am pregnant and i know sometimes other women can tell just by looking at you...i have had light symptoms but am very argumentative of them being pregnancy symptoms since there are almost like period symptoms. my tummy has gotten hard by my ribs and my belly button. when i press on my belly to feel how hard it is it feels sensitive and painful.. my breast have grown but not sure if it has to do with the weight gain.. i have taken home pregnancy test but all of them have come out negative..i want to wait before i go to the doctor when i have money for an ultra sound because currently since im not a full time student i dont have medical insurance and yes me and my boyfriend have had unprotected sex for a while...i want to get other peoples opinions before i decide to go so i dont feel like im just being crazy..also i use to be able to drink tequila without a problem but recently i cant even stand the smell and just a drop of it on my tong makes me want to vomit. i remember that on december i felt flutters just bellow my belly button and thought it was just gas or muscle twitches but it went on for a couple days on and off...and now i feel movement but i feel it could be gas...yes i have been constipated and my friends say my taste buds have gone wack because i have not liked certain things like i use too..i normally pee alot but now its been very often...my lower back has hurt and not so long ago i had horrible pain between my rectum and belly button...i know there are women who get their periods all through out their pregnancy but i hear its very rare....do you think i could be pregnant please leave mature answers because its not that i am being stupid its just that money is the issue so i dont want to spend it for no reason.