Transmission shifting at higher speeds?

97 chev k1500. Automatic. The tranny started running up to higher rpms and speed before it will shift into the next gear. Just as if I was putting my foot down hard on the gas. Moving the gear shift doesn't change what the tranny does. Doesn't "seem" to be slipping. Checked the fluild, a little low... topped it off and ran it awhile, still doing the same thing. Haven't had it do it at all before and now it is constant.