This sounds like an issue with jumping or throwing too much weight into it.
There are a lot of troubleshooting tips for inverting:
-Bring your hands lower, like around face level, as this will make it easier (trust me, it does)
-You may swing your leg, but NEVER jump, even if it's a tiny bit. Never.
-Be sure to pinch the pole between your side and your inside (or pole-side) arm, to give you better grip
-Inverting will take time! Be patient, and if it hurts, back off and see what you can do otherwise
-Be sure that your hips are in front of the pole before commencing the actual invert part. As I said, leg swinging is okay, so you MAY wish to swing from a slightly less-than-in-front-of-the-pole position (sorry, it's late, I'm not very wordy right now! haha)
As for exercises, I would recommend two: First, lots of core strength. Don't just focus on abs, but the whole core. Transverse abdominus, back muscles, obliques, the lot. They will ALL help you in this endeavour. Really good one for this is Veena's free 30 Day Ab Workout. Seriously.
Second, is sort of... leg lifts? You lay on your back, with your back flat on the ground, and lift your legs straight up, so that they point toward the ceiling (your body would be making an L shape). Then, push your feet up toward the ceiling, without using your hands. Your hips will lift, and at first, you may only be able to do just a bit of a pop. Try to work slowly, controlled, and with time you'll find that you can lift higher, for longer. This will give you the strength and control for the second part of your invert, so that you can push your legs up while also hanging from your arms.
As I said, if it hurts, back off the move. This goes for pretty much any move; having some pain from gripping the pole is perhaps acceptable (depending on what kind, and how much stamina you have in the way of pain), but muscle pain is NOT good. You need to take it SLOW.
And, good luck :) It's a big milestone!