I got this crib mattress information from here:
http://www.creative-baby-nursery-rooms.com/baby-crib-safety.html
An ill-fitting baby crib mattress can compromise baby crib safety. Baby's head can become entrapped between the side of the crib and the mattress resulting in suffocation. To prevent this ...
- The baby crib mattress should be firm with squared corners.
- The crib mattress should fit snugly (less than two finger-widths between mattress and crib sides.)
- The baby crib mattress should measure at least 27 1/4inches by 57 5/8inches. Depth should be no more than 6 inches.
- Remove all plastic covering. Never use plastic covering as a crib mattress protection. This poses a suffocation risk.
- Consider an organic crib mattress for peace of mind.
Your mattress doesn't sound right. You should get a new one but make sure to get a refund on the old one. By law it should meet the above minimum measurements.