You're just probably nervous and trying to get into your grove by sending down strikes... so good, that the pitch(s) are sailing right over the center of the plate, right smack-dab into the meat of the bat.
As the season goes on, and you see more and more action, things will settle down for you. I've seen this a lot in my experience and it's not a bad thing. As you mentioned, in later innings you seem to settle down and hold your own.
Remember this though, you're going to get hit when facing the top of the order - that's why those guys are in the top of the order. As you gain more experience you'll figure something out. You'll see. You'll do just fine.