It's true in that usually it's considered acceptable to have a LP ranging from 12 to 14 days, but anything regularly shorter than that is of some concern. But if you think that your LP may be on the short side, the best way to determine it is to chart, use OPKs and generally get familiar with your cycle. If it took 5 weeks for your cycle to complete, though, it could be that you O'ed a little earlier than you think you did.