Totally normal. When they say that most babies start to sleep through by 12 weeks they neglect to tell you they start waking up again a month later. Things like teething, learning how to roll etc all affect the babies sleep. If you are following their signs eg. feeding if they seem hungry, it is very likely to be unrelated to anything you are doing. You just have to ride it through and try to catch up on your own sleep wherever you can. This may sound unhelpful but coming to terms with the fact that babies just don't usually sleep through until a lot older and that it is not related to how good a parent you are, can in fact be the most liberating thing when getting through difficult sleep periods.
Big hugs.