In a normal usage, low storage warning comes up when the internal memory is getting nearly full. The most likely reason for this is the applications count is on the rise over a period of time. To know what are the applications that occupies more space, you can goto Settings->Applications->Storage use. You can see entire applications that are installed on your phone (both pre installed and that you had installed). Together with the name of the application, you can see its size.
How to get rid of this warning:
The only way to get rid of this warning is to free up the internal memory. This can be done in several ways. The best would be to investigate the above said list to spot the applications that you do not need and uninstalling them. The next best option would be to move the application to your SD card. Few applications, they themselves does this during installation and few does not and requires little force. Among the later there are few which are stubborn to work only on internal memory.
There are apps like Link2SD, Apps2SD etc. that will enable you to move apps from internal memory to SD card. I use Link2SD and I would recommend it. It is best to try the application after moving to SD card for its proper working. The system applications are best to be left on internal storage.
SMS/Text messages and contacts do occupy some memory but in par with the applications, their foot print is quite small.
Could you post about the contents of those text messages? Sometimes service provider do send few text messages for APN settings.