Using 3G when connected to wifi iPhone

I have an iPhone 3GS that I got in Oct 2010. I havent had any problems with wifi until now. I had the 200MB data plan and noticed I was at 450MB two wks ago. I knew there was no way that could be right as I havent used more than 65% any month. So to avoid two $15 extra charges, I changed to the 2GB plan and I updated my phone to version 4.3 to see if that would fix the problem.
I figured out yesterday that my phone is using 3G when Im on wifi on my home network (nothing has changed in our network settings). I know Im connected to the network because I can use the Air Video app when on wifi but cannot on 3G and it was working fine. I reset my network settings & reconnected to my network. I checked it again today and its still using 3G. Any ideas?!