Because you are very sad so that the tear keeps pouring out. Sweetie, what makes you so heartbroken? You can take a deep breath and forget all the things. I suggest you to hang out with your best friends. You can go shopping. You can go hiking. Or you can just walk and talk with somebody in the outside. I think this will make you feel better. Burning eyes caused by crying will go away a few hours later. It is caused by straining or rubbing your eyes during the crying.
Maybe you just had an eye infection during the cry. We often rub our eyes when we are crying. And we also need to wipe out the tears at this time. If your hands or the handkerchiefs are very dirty. You may get an eye infection. Or maybe your eyes are very dry after pouring out too much water. Your eyes will produce more water to relieve the dryness caused by crying at this time. You can apply some artificial tears to lubricate your eyes. Or you can apply some chilled cucumber slices on your closed eyes to calm down your irritated eyes. And drink plenty of water can help a lot too. Good luck!
I think it is common to have watery and burning eyes after crying. It will take you a few hours to calm down your mood. As to your irritated eyes, it will take a few hours for them to recover. But many people will have puffiness under their eyes after crying. You can use chilled tea bags to relieve the puffiness. Good luck!
You can apply some artificial tears to lubricate your eyes. And lie down to have a good rest and don't try to think about the sad things anymore. Just think about something that makes you happy. You can drink plenty of water at this time because your eyes need water after crying. You can also place a piece of ice on your closed eyes. This will help calm down your burning eyes.