Yes, this happened to me after my first birth. I noticed it right away, but didn't mention it until about 2 weeks post partum. For me it would alternate between locking and just giving out suddenlly. It really hurt! At first my OB said "honey, you pushed out a 9.5 pound baby, you're going to hurt!" but finally 4 months later I went to see a different dr. I got all kinds of referrals, xrays, went to an orthopedist who sent me to a physical therapist, who sent me back to ortho who diagnosed me at first with bursitis and gave me two corticosteroid injections into my hip (OUCH!). It didn't help.
Then I got pregnant again, and it was after THAT birth that I put my foot down and said "Find out what's wrong with my hip! I'm young, I'm active, I shouldn't be in pain." More xrays, saw a chiro this time, more physical therapy, finally did a flouroscopy and discovered a labrum tear. Labrum tears are very common in shoulders, and easily fixable, but not common in hips and not so easily fixable either! Through flouro, they shaved off a piece of the jagged edge to see if that would help, then sent me for a contrast cat scan to look for more tears. Discovered a lot of micro tears in the cartilage around my whole pelvis from the two years of imbalance I had had (and I was active duty military this whole time, so running daily on the injury). Another corticosteroid, lots more chiro visits, and then since I finally knew what was wrong, I could focus my workouts on strenghthening the right core muscles to allow myself to heal better. I got a personal trainer and worked really hard. I had pretty bad symphasis pubis diastasis with my second pregnancy, so I had to heal that, too.
My pelvis was a mess for a while, but it did get almost 100% better with a lot of work!!! Of course then I got pregnant again, and my pelvis has really hurt this pregnancy, but my hip has NOT!!!! So I'm hopeful that the pain will go away MUCH faster this time, and this time I know what exercises to do to help the healing process.
SO...all that, and you're only a week or so post partum, so maybe don't get too freaked out yet. Consider a yoga class soon to start working on your core muscles, consider seeing a chiro, and if it's still hurting after a few weeks, see your dr.