I think both your FP and LP can vary. I know my LP was always the same (14 days) but it was my FP that was a) a bit shorter, and b) varied a day or two from cycle to cycle.

But I'm not sure how common this is, from my experience you seem to see a lot more girls asking questions about their LP because it is short or changes from cycle to cycle. So I think changes in LP are more common?

From what I understand it can be more of an issue if your LP is the one coming up short, as your body needs the 14 or so days to send the egg out and give it a good chance of implanting successfully. Having a short FP is not great but I think it is less likely to cause an issue in that cycle.

So you may need to track a few more cycles and see if you can tell which phase is shorter. I was able to pinpoint my short FP fairly easily without temping because I get pretty obvious CM and other signs. Sounds like you can probably pick it pretty well also. Of course the other option is you get pregnant this or next cycle and you don't have find out