Hi Holly - as far as I know a 21 day cycle shouldnt' be a problem in theory - as long as you ovulate early enough within that 21 days to mean that your luteal phase is long enough. A short luteal phase can make it harder to get pregnant, particulary if it is shorter than 10 days. SO if you ovulate on day 7 for example, then it shouldn't be a problem.
There are several ways you can find out when you ovulate - I think the most reliable is to take your temperature every morning, as the only to know for sure if you have ovulated is by the temperature shift. However, using OPKs or a fertility monitor can also help you to understand your cycle more clearly, although they cannot tell you for certain if you have actually ovulated.