That could be any number of things, quite frankly - but here's the thing - we have absolutely no information on which to base our advice! We need a diary from you so we can see things like what you're growing, what you're growing IN, how old is the plant, what nutrients are you giving and in what amount, how often/much are you watering, how far is the light from the plant, what temps/humidity is she in... you give us a picture and ask what's wrong... that's like taking a picture of a red spot on your arm and sending it to your doctor asking "is this measles?" ...
Do yourself a favor here... join the community and start a diary - we ALL want to help but with no information, all you're going to get are a bunch of guesses. So sorry, mate!
Good luck!