After getting the flu shot on the next day can i continue taking diet pills or do i need to wait?

OK to continue: There shouldn't be an interaction between the various flu shots and "diet pills". But the bigger issue stems around diet pills. If they are not prescription pills being monitored by a doctor, I would urge you to consider avoiding over the counter "diet" pills. For many there is more potential for harm than good. Best of luck.

Perscription meds?: If you are talking about medication prescribed by your physician, yes. If your pills are over the counter, these may contain large amounts of caffeine - I'd advise holding off for at least one week. You need to eat a balanced diet, not fast after Immunizations, so if you are skipping meals on any type of Diet pill, this will not help you have a healthy immune system.