I've had my period for 3 weeks now & I'm starting to get worried?

I'm 21 years old & today is officially the start of the 3rd week of my period. In the past year I've gained 20 - 30 pounds & my period had been a little irregular before the weight gain but after i gained the weight it became more irregular, was getting my period every 2 months or three but the last time before this I got my period in January & then 5 months later(Tracking it in my Iphone) Bam it came(In the past 2-3 weeks i've lost 10 pounds) the first few days it was very light & then the 3rd day it was a heavy flow but after the 5th day it didn't stop(I usually have it for 5 days) by the second week I thought maybe because I haven't had it i awhile Its gonna last longer this time around but I was wrong.Last night I wore a pad to bed but when i woke up there was no blood at all so I thought yes its over! So i got out the shower & got dressed for the day(without a pad on) & went out & the when i came back I realized that there was blood on my underwear so now i'm a bit worried. I'm just thinking because I haven't had it in 5 months this is why this is happening but i don't know. Can someone give me some real advice or insight as to why this is happening to me?

Get it checked by the doctor. There could be something wrong here if this is the first time this has happened…