foamy gear oil??

I just changed the gear oil in my axle. When I pulled the cover and the oil started running out it was okay but after about 3/4 of it drained a lot of foam started running out. When I pulled the cover the differential, ring gear and housing were covered with foam. I had only driven it about 2 miles and it sat for probably 45 min. before I pulled off the cover. Is this normal or should I be worried? I only ask because I never noticed anything like this before on any other cars I've changed gear oil in.

thanks,
Josh