very young and idk if im pregnant or not

here i go again...well im really scared im 17 and havent had a full period since may like 5th? i did have a boyfriend and yes we had sex we always used a condom tho but alot of the times we had sex we were drunk but still used condoms well after prom may 20th and we had sex that night i havent got my period still and about 6 or so days after we had sex that night i had pains in my stomach and i had brown discharge like no blood for around 2-3 days about a month later i took a couple pregnancy test and said negative but idk if it was too early i then went on diet pills (oxyelite pro) and i work out at the gym like 2 days a week and i have soccer for 2 hours 2 days a week so im pretty active but i did lay off the diet pill one day and got the worrseeee pains in my stomach n some blood so i was yes my period n txted my boyfriend(hes an ex now) and said got my period bc we were scared but then it just went away like a couple hours later like the next day it was compeltly gone but i was sooo bloated so i started the diet pill again then i went off it for 2 weeks still no period at all and just started it back up but i havent had a legit period like a full one for 3 months now and im very worried im also very gassy and i feel weak alot and constipated sometimes but im tired alot idk if im active so im not getting my period or im pregnant bc i got random brown **** or the pills? i CANT tell my mom im way too scared and im too nervous to buy a pregnancy test idk what to do help...