What Causes Irregular And Heavy Periods?

i had spotting here and there, for about 3 days and there until Wednesday the 28th it came down and it wasn't so heavy.it was normal like.but now on my 5th day its heavy my pad gets full.a soon as i stand up after changing i can fee it come down.last time i had my period it was the oppitse...it came down regular..then when i stoped i had spotting that came into last month..i did have a minor tract infection that i didnt know about becuz i had no pain but was giving anit bodies as long as pain killers for something else..but that was like amost 3 months ago....idk whats wrong.. iam 18...im 5'6 and i way between 155-160