Is this pregnancy symptomz?..or am i just imagining all this?..please help URGENT?

ive TRIED with my boy frnd on december , but i did get my periods on january and feb this month, march i havent got it...so im so worried =(..i havent had any signs of pregnancy, but 2 dayz back i constipated and today i did...so im more worried, i dont have nausea or headaches , except once i vomited and had migraine. i have gastric problem past a week.my tummy is all chubby bt not hard and round, its the 16th week, i should have gotten my periods on 9 th of march, its a week late...im soo worried. please help. its urgent. i have back pain not extreme but yes i have it, and sumtimes my tumy hurts for a while then its ok. please i need some good advice, keep your rude comments to yourself.