Opk question

Calie

I had a baby 4.5 mos ago. I am pumping and bf, so i have had a period about a month ago. So my hubby and i have been frisky a few times. I am taking OPK kits to try to avoid peek fertility, and i get a pale control line every day...for 15 days....with my daughter it went up to peek then back to nothing and now always pale pink lines. Anyone know why?