Hi Nikki11
The pain could have been ovulation pain - this seems to be a quite common occurence. Not sure about implantation pain as you would potentially only be on CD18 - which I think is more likely to be ovulation.

The best thing you can do is check your Cervical Mucus - if it is EWCM or sticky then this indicates you were fertile around that time which would indicate ovulation again.

Basically it is difficult to say without you charting (temps or CM) - however wait another week and do a HPT and see. Hope you get your BFP real soon. I have irregular cycles and have found that charting has made what my body is doing a lot clearer.

Good luck.