Why is my singing voice changging sooo much?!

My singing voice used to be so awsome. I could reach really high notes and sing whatever I wanted without it cracking. Now it cracks when I just sing a low note. :( :(
I have been drinking lots of water, not straining it to much and been warming it up before singing.
But you know whats weird? It comes back when I sing a choir song when iam in the music room with the choir. A couple of hours after or before it isnt normal it cracks. But when I am singing in the music room it's fine and it doesnt crack or anything.

Why?

Talk to your instructor about it. That's a music professional who can listen to you and give better advice...

One of the most important things for a singer is to be able to hear yourself while you're singing. Singing creates a feedback loop where your hearing simultaneously listens to your voice and corrects the pitch and volume. Your music room most likely has good acoustics and a low background noise level so you can hear yourself well. Singing with a chior provides additional feedback from the other singers. However, if you can't hear your voice or it's mixed with a lot of background sound there is a natural tendancy to try to sing louder to be able to hear yourself better. You may be forcing your voice beyond it's limits, causing it to crack.

If you're serious about singing, pay close attention to your hearing and your ability to hear your own voice in places where you agree to perform. Being chosen to sing the national anthem at the beginning of a sports event is an honor but it becomes a nightmare once your voice starts bouncing off all of the walls of a stadium. Your issue could be more about where you're singing than a problem with your voice. You have to be able to hear yourself clearly to produce sounds at the exact pitch and audio level. That's easy in a low noise, reverberation-free environment but very difficult in places with high levels of background noise.