Having a baby can cause all sorts of changes. They usually go away with time. Anxiety also cause a lot of things to go awry, as it makes it harder to relax the muscles. But it really could be all sorts of things. Tension headaches, sinus headaches, vascular headaches, sleeping wrong, not sleeping enough (having a baby can do that), not enough hydration. Can't know what's causing it, but since it happened after pregnancy, I'm guessing it was from that or pressures and changes in habits caused by that. If I'm right, and I have no idea if I am, it should level out. One thing you might try, which is very hard when you're a newish mother, is meditation -- can do wonders for headaches. I'd hate to see you give up running, so I'd rather see you find a way to deal with it rather than doing that. Have you altered the time of day that you run? Could also be a factor.