It's possible that the mid cycle thing delayed ovulation and your period is now due a bit later.

Also Different tests will show results at different times. I had a early + with no1 about 5/6 days before period due and it was pretty clear +. This time i still only had very faint + the day my period was due. Both were just as sensitive as eachother according to the packets. I stopped testing after that as + is + as the doctor said. She didn't even repeat the test when i saw her as i had to go to the doctor the next day anyway for other reasons.