Hi Ganesh,
This normally happens when Google servers notice high amount of traffic hitting their servers from your Public IP address. Most probably due to sharing of public IP between other devices. Most of the customer do not use a separate public IP for Proxy and use default NATing rule with other devices sharing the same IP address. All the traffic using this public IP address and generated towards Google servers will be getting counted. So to rule out whether traffic from proxy alone is causing this, you can change the public IP used by the proxy. If the issue is still happening (i.e. even after isolating the proxy), then it is due to the amount to requests towards Google via the proxy. After confirming that this is genuine, splitting between multiple IPs could help