Help! URGENT please! Proof or gaurdian?

My periods started on 18 Jan and ended on 23 Jan. I had protected sex on 27th Jan although I'm little unsure of few things. My partner ejaculated on my thighs near vagina but didn't touch it. We wiped it carefully. The intercourse was protected. My lower abdomen was paining and stomach bloated on 1st Feb (I didn't eat that day) . I took pregnancy tests on 2nd Feb evening and 3 Feb morning. Both were negative. On 3rd Feb, I had dry brown blood in my underwear. So I took 2 more tests. One on 4th afternoon and other on 5th morning. Both negative. Today and yesterday I felt urge to urinate (8 times). I eat a little more these days and my mood swings. My tests are -ve but could it be that my hcg levels are low or it could be showing wrong for other reasons? Could I be pregnant? I am 17 and it's hard for me. Please help. Should I go for ultrasound? Does ultrasound need age proof or gaurdian? I have some friends who've studied medical science and completed their UG. HELP URGENTLY please!