whats a good App?

my husband and I are TTC.I had my IUD removed last Saturday and no bleeding for a couple days then started to bleed kinda light then a little heavier. I thought it was my period then I looked online and said it was from my IUD removal and my body was catching up from the removal. So I dont know my last period was cause I had that IUD for over 4 years. what is a good app I can use to tell me when the best time to have a baby when I dont know my last period.