What Are The Chances Of Pregnancy With Non-penetrative Intercourse?

Question : 7 and a half weeks ago I was in sexual contact with my boyfriend. We were both naked and he was rubbing his penis against my vagina, but there was no penetration. This was two days before my period was due.
Two days after this my period came, so right on time. It was completely normal, lasting around 5 days and a normal flow.
I was not in any sexual contact the next month.
28 days after the period just mentioned, my next "period"came. However, unlike my normal period I had a light brown discharge instead. This lasted five days. On one evening I went to the bathroom and there was normal blood, but this cleared and the brown discharge continued. There was normal blood again one morning, and again it cleared and the discharge continued for another day or so. I started to get concerned cause I read that this sort of "period" could actually be implantation bleeding!
After this "period" finished I had a persistent pelvic and lower back ache which furthered my pregnancy concerns. This pain lasted about a week and a half and now seems to have eased.
I took a urine pregnancy test a week and a half after this "period" started and it was negative.
However I am still concerned.
A week and a half ago (which would be almost 2 weeks since the odd period), my breasts became quite tingly and sore. This went on for a few days then stopped.
A couple of days ago I became quite nauseous (though I haven't felt nauseous today), and today my nipples became tingly and sore again! I am also hungry all the time, no matter what I eat, and I eat a pretty balanced diet. Please help! Could I actually be pregnant, and if not, what caused my last "period" to be so unusual? Any advice is much appreciated.