Every poker decision is a new decision. Once your money is in the pot, it belongs to the pot. You need to count all the money from everyone's bets and whatever is in the middle already then compare that to how much you need to call.

Examples:
Blinds are 100/200
You are sitting on the button.
Everyone folds to you.
The pot is 300 and it costs you 200 to play your pot odds are 300/200 or 1.5:1

You call, SB calls, BB raises it to 400.
The pot is now at 800 (200 from you, 200 from SB, 400 from BB) and it will cost you 200 to call.
You pot odds are now 800/200 or 4:1 (slightly more if you believe the SB will call as well)