Hi, I'm having trouble with hitting low note on pitch. Sometimes it sounds so rusty and doesnt even sounds like singing. 
Sometimes i can and, the other times i cant sing the same note.

Would this because my voice isnt warming up properly or not hydrated enough? 

if its because of warming up what exercise do you recommend?

Thank you :)

It's totally normal for your voice to start "frying", as your vocal fry or pulse register is just below your chest/speaking voice. 

Hydration is always a good idea! But I recommend using something closed (like a lip trill, NN, NG, MM, etc), a narrow vowel (like EE), or a wide vowel (like the A in CAT). Switch to one of those configurations when it's starting to feel too low!