Hi there Sarras,
Glad you caught me before I posted. Yes, check out how many mg of niacin is in it. If it's more than 50 mg or if you took it without a full meal, it could do that. And the other two possibilities you mention also sound feasible. Good idea just to stay observant and mention it to you doctor if it does not go away.
That flu has been a terrible thing this year. I'm glad you're on the mend.