You asked: I thinking too much into it because I have such an issue with asking for help?
Yes. She's a grandmother for heaven's sake. Although she might not be goo-goo ga-ga over babies what you're asking is not unreasonable, not an imposition, and she can always just say no. You have delayed quite a bit in asking her though - is that because you just found out about the event or have been putting if off because your anxious? And if this kind of thing makes you anxious, why can't your husband to the asking? The words are pretty simple - "thank you so much for baby-sitting last weekend, we had a great time at our event and hope you had fun with little Johnny. This is unusual for us and last-minute as we don't go out often at all but we have another event this weekend and we're looking for some help with Johnny again. Would you be able to watch him again? If not, it's no big deal but we figured it doesn't hurt to ask." And then she either agrees or not.
Have your husband pick up the phone and ask her right now. No sense spending any more time or energy on getting anxious about this!
As far as accepting help...realize that by accepting help, you are allowing others the same opportunity to feel good that you get when you help out with something. I was on bedrest for two weeks with my last pregnancy and the thought of being waited on hand and foot was mortifying, but my friends LOVED banding together, swooping in and taking care of my family. People were coming out of the woodwork to bring us dinners, drive my kids places, drop off muffins and other goodies, etc. It was humbling and wonderful. Another friend (one who helped me in the past) had a terrible bike accident a couple of weeks ago and our group of friends geared up to bring over dinners while she recovers. You had to respond quickly to claim a night because everyone wanted to help. Helping people is rewarding and fun and builds relationships. Allow other people to have those positive experiences by allowing them to help you when you need it.