There are many possible reasons.
First of all, check if the solution contains too much salt. The density should be no more than 0.9%ã€‚
In addition, eyes are very sensitive, so you need to get used to salt water. Try again when you feel burning, as burning sensation is normal for the first time.
Lastly, if the pain is constant, you may have open sore or little scrapes in your eyes.
Or you eyes have infection or conjunctivitis under worst condition.
Ok, it seems that you got some trouble. It is true that salty water may help with our eyes when we got something inside. But excessive salt may give rise to irritation with our eyes. The proportion must be controlled well, maybe you have put too much salt in the water. Just be careful and flush your eyes with mild and clean water, and get some eye drops to treat your eyes.
Although the salt water will have the role of getting rid of the infection at the eyes, it could make your eyes get burning because of the stimulation. You should wash the eyes carefully with the clean water then. At last, you use the clean cloth to dry the eyes. You could also do the warm compress to make your eyes feel comfortable.