Using ovulation strips

Heather
For those of you using OPKs, do you get a positive on the same day every month? Mine have been jumping around. This is my 5th month using them. 
Month 1: positive on day 15
Month 2: day 15
Month 3: day 16
Month 4: day 21 
Month 5 (this month): got an almost positive on day 13, followed by a lighter line the next three days. Wondering if I ovulated early this month. So confused.