It might be and it might not. I've heard of some girls on here who've gotten BFP with LP of 9 or 10 days, but it can be a problem. It just depends on how long it takes the egg to travel to your uterus (5-7 days I think is the norm) and then how long it takes to implant. If this is a pattern you're seeing over a few months of charting then if it were me I'd see a specialist or look into one of the supplements that can lengthen the LP. I had that problem with my first month charting, 9 day phase, and it stressed me a bit, but the last 3 months have been 12 or 13 days so I think it was just one weird month or maybe I had incorrectly calculated the ovulation date that month.