Suggest Treatment For Lower Back And Pelvic Pain

Question : So I have had a sore back and pelvis for about a week and a half now. At first I thought it was PMS. My period that then came was very unusual. It was on time (28 days after previous period) but instead of my normal flow it was a light brown discharge which came. This lasted five days. There was blood two of the days but I only noticed it when I went to the toilet. After the bleeding stopped the sore back and pelvis persisted. I became very concerned that I had actually experienced implantation bleeding and
was pregnant.... I was in close sexual contact with my boyfriend 4 and a half weeks before this unusual period (he did rub his penis against my vagina with possible release of pre ejaculate fluid, and he ejaculated close to my vagina but there was no penetration). After this encounter a completely normal period came on time two days later. However despite the low chances of pregnancy I remain concerned. I took a urine pregnancy test yesterday and it was negative but everywhere I look I see implantation bleeding being very similar to my "period" just passed, and the fact that the pelvic and back ache are still here concerns me. The pelvic pain has appeared to be worse after eating. It can have a sharp, stitch like sensation to it at times, but at other times it can be dull, and after eating it can very much resemble menstrual cramps and then take on an almost bubbly sensation as if bubble wrap is being popped in my abdomen. When I have woken up
the last few days my sides have felt like they have a bad stitch, though the pain seems to die down once I'm up and about, then comes back after eating. My back ache is lower back ache which is a persistent dull ache. It also seems to spread up and I now have a bit of neck ache and have also noticed intermittent pain near my armpits. I am really starting to get concerned and I just want to know what is going on. I really just want to know what has caused the unusual period and why I am still experiencing pelvic and back ache when I don't normally. Is there still a chance it could be pregnancy despite the negative test and normal first period and not actually having penetrative sex? Any advice is much appreciated