what causes my bladder infections

i had my first bladder infection mayb 2 months ago an this mornin i woke up an i knew i had another one because of the same symptoms i had to go so many times ..so i drank like 2 glasses or 3 an then went to the washroom an more an more came out i never felt pain jus had to go alot but after drinking alot of water the urge stopped...and im thinking how i can get them so fast ever since i was little i never drank alot of water im 19 now the only thing i can think of is sex i jus lost ma virginty an got it maybe the 13th time i did it...wer always protected so wha causes them to happen so often?...someone plz give me advice