opk+ then opk- in 10hours?

Aimee
This morning I used my SMU and got a positive result (top-there's 2 tests but they were taken at the same time this morning, just 2 did brands) at 800am. 
Now I just got home from work, and tested again and I'm getting a neg. (bottom) is this normal? I am on cycle day 13 and we BD yesterday morning and plan to tonight as well. Is this normal for levels to change this quickly? I know I'm not out and that the positive only means your probably going to drop the egg within 12-24h,  just looking for others opinions.