The detective noticed John's little smile, almost a smirk, he knew John had spotted him behaving correctly. He awarded himself a point for pleasing John in this way, not that he kept count, but they had this pleasant little competition going. John got points and a smile from Sherlock for getting one over on Mycroft and John scored every time anyone complimented him on his blog.
Sherlock smiled his Just-for-John smile at his best friend with his fork halfway to his mouth “You're on sparkling form tonight.” and put his fork down, feeling queasy. “I'll get us another beer on my way back” he coughed coyly “from seeing a man about a dog,” and left the table for the men's room at the back of the restaurant.
Feeling less than fine Sherlock made it into the single male toilet in time to lose his dinner. This shocked him. He was never sick. He swilled his face with cold water and patted it with a paper towel as he stood looking at the pale reflection in the mirror.
He got back to the table, realised he had forgotten to ask Billy for another beer, covered his confusion and went back, feeling very disconcerted, to the counter. He didn't want John to worry so when he got back to him he plonked the bottles on the table, sat down and tried to eat a bit of cheese at least.