I would Check to see if it threw a code, do the stomp test, check trans fluid too. Of you change fluid and filter, it will help this issue. Speaking from experience. My car does it too. Mine has a code for incorrect torque converter lock up or something. Mine is a four speed and slams into 4th, and did the whole downside and stay in third at 60+ coming home from work today.
2003 GMC Sierra SLT 2500HD CC/SB