Sounds like she has stopped grooming herself as thoroughly as she used to and she just has these "clumps" of hair that didn't manage to complete shed off.

Poor grooming habits CAN be an indication of underlying medical issues, however, such as thyroid issues, diabetes, or just arthritis. We will also see this in pretty obese cats because they just can't physically get to the areas they need to.

In either case, as mentioned, these areas are likely matted and can be quite painful/uncomfortable for her and something needs to be done. Even if it isn't quite matted yet to the point of being uncomfortable/painful, you need to start working them out to prevent them getting to that point. The glove Karen mentioned is a great idea.