In this sentence 'am' is expressing 'wanting' so literally it would be 'You were not wanting to drink the lemonade'
For the sentence 'You were not drinking lemonade' you would need to replace the 'am' with the usual 'yn' connector.
Do't ti ddim yn yfed y lemonêd
For the positive 'You were drinking lemonade' you would use:-
Ro't ti'n yfed y lemonêd