Confused. Please help!

Steph
My period last month was a week late, starting in 14th Nov and finishing on 21st. Since it finished, I have been taking OPKs, all of which have been negative. I have three apps which all say different days for my fertile window and the due date of my next AF (my average cycle length is about 31 days). A few days a go (CD20) I had a sharp pain in my left side, I've never experienced this before and it was enough to make me need to sit still for about 10 mins! I have also had a little spotting. 
I'm so confused as to what's going on. I'm going to see a Dr but we have only beenTTC since August so I think they will just say I need more time off the pill, even though I'm usually quite regular since coming off it and this is the first month I haven't git a positive OPK at all. Thank you for your help xx