When is my AF due??

So I stopped my birth control shot back in Sept of 2015 and I did not get my first period again until February. So then in March I got it on the 11, April the 12, and May was the 16. So for June glow is saying my period week starts on the 20. Shouldn't it be the 16? I know it's only four days but I really would like to know when would be best to test and I don't get why glow made it four days later than last month.