I'm currently on depo injections. I have missed my period. Could I be pregnant? I have taken a test but it was negative. The only change is my eating.

Pregnancy unlikely. Pregnancy tests are very accurate and not affected by foods, herbs, common illnesses, sexually transmitted diseases, or medications. A correctly done urine test gives a reliable result 3 weeks after sex (3 weeks after when conception might have happened) or 1 week after missing a period. If done sooner, a positive result is likely real, but a negative result may be due to testing too soon.
Likely a side effect. When you take depot progesterone injections it is very common not to see your period. It thins the lining of the uterus. It is very important to take your shots strictly according to schedule - for instance Depot Provera should be given every 12 weeks. None of the contraceptives are 100% effective, thus if you are worried about pregnancy it is important to test.