You are probably being hit the price increase that vhi announced last year (see [broken link removed]). As for why, the official reasons are outlined in that link but it's basically more money going out than coming in.
There are plenty of other health insurance products out there. A good place to start would probably be the Health Insurance Authority's website, which has a number of different comparisons, see [broken link removed].
Also do a search on here, as there have been a number of threads over the last few years about switching plans and/or insurers.