Opk question..

So I am new to using the opks and thought I had a positive but turns out that it was like two days before I ended up getting my period. Unfortunately I have been very irregular since I came off birth control in the end of August, I have had a 38 day cycle and a 49 day cycle. I'm not sure if it was a true positive or not, seems like I haven't had any come back where it was obvious- I suppose I should buy some digital ones. Has anyone else had it come back falsely positive right before a period?