I have to be careful with those cheap tests - they are cheap for a reason!
I suspect you didn't ovulate when u think u did and are now having another surge. How dark did your opk get earlier on your cycle to make you think you ovulated? You would be better using the digital clearblue ones.
I can't explain it but I get it too. I was curious one month as was really in the mood for sex on about CD3 (had 36-48 hour periods so had stopped bleeding by then) and out of curiosity did an OPK which was almost positive. Don't think it's uncommon at all.
I noticed a few cycles ago I was getting a very strong line when af was due. I asked the same question and a few people told me that it's completely normal to get a surge right before af (some also see fertile cm).
In my case it was just very very late ovulation though