Could be irritation due to the spermacide. Some of them are REALLY harsh on the vagina, even causing little itty bitty open sores. Whish would be uncomfortable and, as healing wounds do, itch.

I suggest using a LOT of lube when having a lot of sex. Then again, I suggest using a LOT of lube always. But my guess here is that it's the spermacie irritating you.