Don't you just love it when what you think is going to be just regular sex turns into something hot, intense and animalistic?
The experience you described has happened to me (being on the receiving end of it.) Sometimes it's a matter of thrusting at the wrong angle or too much lube causing slippage. As long as it isn't painful, I just go with it; making sure that my lover's fingers fill my vagina and pleasure me that way, as well.
When men are younger (or just plain ignorant) they'll plan for the "accident" to occur, thinking that once he's in, the lady will be fine with it. This trick rarely works out in his favor and he's likely to get slapped or made to sleep on the sofa for a month, if not kicked out of the house entirely. A male friend of mine firmly believes that every man who wants to try anal sex should have a strap-on used on them first so they know how it will feel to a lady and it will make him take his time to relax her and slowly stretch her before penetrating her. I think a man should be open to having anal play reciprocated so he knows what it feels like and how easy it is for things to go wrong and become extremely painful. Because when it goes wrong, the expression of pain being visceral truly applies so it's best to take it easy. At first, anyway. 😃
The first time I attempted anal sex was terribly painful. My boyfriend penetrated me quickly with his full length and rammed me deeply. It was an accident. I was lying on my tummy in the bed and he was kneeling behind me. We had no idea how quickly things move and how easy it is to lose control when the anus gives way and opens up for a lover. I screamed, "Whoa, Steve!" I cried that, "It hurts. It hurts. It hurts." He started to remove it quickly but that hurt, too, so he had to remove himself millimeter by millimeter. Then I curled up into a ball in the bed while he held and kissed me. He was really good about the whole thing.
Although my first experience was far from pleasant, my interest in anal play wasn't waned so we attempted it successfully the next weekend after my bottom had a chance to recover. It was divine. I always took enemas before we went out so I was always clean and ready for anything; a regimen which I maintain to this day. Eventually, we could laugh about our first (literal) stab at anal sex.
For the rest of the time we were dating, we regularly had oral, vaginal and anal sex every weekend. Often several times a night. We were teenagers so recovery time was nil back then. I can still remember the scent of our sweaty bodies mixed with baby oil in the un-air-conditioned room on the second floor of his dad's house.
In my high school yearbook, under favorite memories, I listed several special dates and brief memories and the quote, "Whoa, Steve!" to remind me of all the hot anal sex we had.
I'm still having hot anal sex as often as possible. I'm glad you and your lady enjoyed it and hope you have many more kinky times together.