Annoying keyboard issue

Ok this is driving me NUTS!! I have an iphone 4S running iOS 7.1.2 (but the issue also happened with previous iOS versions). From time to time, when I type, the keyboard will automatically switch from letters to numbers. It is SO annoying!! For example, ill press the "P" button and it will type a zero and change my keyboard to numbers. Or ill press the "T" button and it will type a five and change my keypad to numbers. Also, most of the time even when i press the backspace and try to retype the letter, it will keep changing the keys to numbers....sometimes taking me several tries to get it to come out right. Has anyone ever seen this before? If so, is there a fix? Please help!!