Define 'very fast' - can you provide some specifics. Also, which version of iOS are you on are also have you looked at the Battery applet inside settings to see what your X is reporting the biggest drain to be?
I'll assume that you are referring to battery charge. If so, try to restart (totally power off) the phone first. If this won't help, and you are on iOS 11.3.1, try updating to iOS 11.4 beta. There seems to be a bug with the battery usage and beta usually helps.
How long is your battery lasting? There could be an issue with a certain app.
On your phone open up Settings, then Battery. From there you can show last 24 hours and last 7 days.
If you notice a specific app using a lot of battery you can check to see if there’s an update for it or check the permission settings to see if for some reason it’s using your location when it doesn’t need to.
As @Minorite mentioned, you could try restarting your device to see if it helps.
Also, cell signal could be an issue. If you’re frequently in areas with low signal it could cause the battery to drop fast.
There are a lot of factors involved. You could also contact Apple directly for them to run diagnostics on your phone.