The gas gauge in my 1993 Cadillac Seville is Freaking me out!?

I put $70 in gas in it yesterday, and it read full all the way from the station to home (6 miles), all the way to work and back (20 miles) and all the way to school (6 miles). then when i got back in the car after school, the gauge read 7/8ths of a tank. i was parked on a bit of a sideways incline, and the gas moves, but normally it goes back after i'm on the road. but when i got home, it was at 3/4s of a tank. my gas mileage is better than ever right now, 22 mpg, and its got an 18 gallon gas tank! i checked for a puddle or a leak, and found nothing. please help, i cant afford to fill it up that often