Common problem Holly!
You may live in a hard water area. That sometimes is the cause.
Some times it is just poor plumbing but usually the real number 1 offender is inside the cistern.
You can sometimes replace the internal workings of the cistern and sometimes the entry pipe work but this does not always work.
If you can afford treat yourself to a new loo!
paulonyett@sky.com
Qulaified Sparky and plumber of 20 years
Paul
December 2008