Mega + Ethernet shield + SIM900A shield

[system]

Hello friends! I am in the final stage of my project. I am stuck with a problem.
Here are the components i have

• Mega 2560
• Ethernet shield
• SIM900A shield

SIM900A requires 12V 1A power supply.
I have mounted all these. I have used a 12V 1A external power supply. When i connect all these 3 together the voltage regulator(AMS117) in Mega gets extremely hot and i am scared whether it might burn away! Please help me with what should i do now as soon as possible!!!

[system]

I individually connected mega with Ethernet shield and mega with SIM900... I came to know that when mega and Ethernet shield is connected to external power supply, the AMS1117 of mega is heating up extremely!!! But SIM900 and mega are fine in external power supply... So, what should I do now????