Transmission acting up in first - second gear

What is your question?

1998 Chevy Blazer 4 door, 2WD I was driving on the highway, 70 mph for over 100 miles today. When I took an exit into the city to get fuel, I noticed as I was turning off of the exit ramp that my transmission had slammed from first to second gear. (around 20-25 mph.) This happened every time since that first occurrence, perhaps not as bad toward the end of the trip home.. not so sure. I don't have any issues with this at any other speed so I wonder if it could be a solenoid or fluid. I am the second owner of this blazer. I have not inspected the fluid as of yet, I am sure that the fluid has NEVER been flushed since there haven't been ANY routine maintenance procedures done on this thing until I took ownership. (Plugs, fuel filter, wires, nothing!) Any help would be greatly appreciated!