Does luteal phase always stay the same?

Hannah • TTC #1 since January 2014
This is only my third month off the pill and as I fairly sure I didn't ovulate in month 1 due to a very short and light AF, I'm still getting to grips with my cycle. Last month I had a 30 day cycle and ovulated on cd 16 I think. But this month I got a +opk on cd
​13 so probably ovulated cd13 or 14. Glow then adjusted my fertile window and my cycle length, so it looks like it assumes my luteal phase will be the same this month (15days) and therefor brought my AF dates forward by a couple of days. Problem for me is that I'm desperate not to get my hopes up again this month and so if I'm expecting AF when glow says, but actually my cycle is still 30 days and my luteal phase has lengthened, then I might be getting excited over nothing! Should I trust the app or just wait a couple of extra days for AF?! Help!