Answer please

Okay ladies , I usually have 28-29 day cycles. I have had cycles at 26 days and cycles at 31 ones but it happens like once a year. I started using opks for the first time this month and I didn’t get my first positive until cd 19 around 4pm so I think I ovulated on cd 20. So if I’m correct then I’m 7dpo. Anyways I’m feeling period like cramps. All of the apps I have say I’m suppose to start my period on different days! I’m just confused. I don’t want my periods to be off track. Anybody else have this happen to them before?