Transmission

My transmission was rebuilt about a year back. Well the last week or so I have had to put two quarts of transmission fluid in my car. It was not slipping then, it just seemed line it was grinding a little, so I checked it and it was a little low. I was on the road so I had no choice but to put cheap fluid in it. So yesterday my car had been driving just fine. No slips, no stuck gears, no grinding, no nothing. It ran perfectly smooth. But as my day was closing out and I was headed home I caught a red light. When the turned green I tried to take off and it hardly seemed to go anywhere, like it was still in gear but I had it in drive and it seemed like it was in fourth but actually slower. Then the rpm's would go high like I was in neutral but I was still accelerating at a very slow pace. So I pulled off the road and killed the engine. I waited a couple minutes and started my car back up And put it in reverse, it went in fine. I then put it back in drive and tried to accelerate, and I did. But only for a short second before it seemed to go into neutral again. So I pulled over and repeated the whole process. I finally got home and parked it for the night. Well when I was leaving for work this morning I started it up and tried to drive off. I made it about two blocks just fine before it started putting itself in neutral. I pulled over and tried shifting into all gears, park and neutral was all I had. So I killed the engine then started it right back up. Put it in drive and turned around and went home. It barely seemed to accelerate at all and I was running high rpm's the whole two blocks to my house. So what could the problem be?