Why did i get gastro twice?

Gastroenteritis??: there are tons of bugs, bacterial and viral that can cause gastroenteritis. or, depending on time course, you never cleared it to begin with or had a suprainfection. or you were treated with antibiotics the first time around and your symptoms were really a treatment side effect. so myriad of possibilities... need more info if you want a good answer.

Recuring GI symptoms: Lots of things can cause GI distress to recur--incompletely treated infection (e.g. Giardia) or re-exposure with re-infection; maldigestion (pancreatic insufficiency) or malabsorption (celiac sprue); food intolerances with re-exposure to the offending substance (lactose, sorbitol, etc.), irritable bowel syndrome, missed diagnosis (incomplete evaluation), non-gi related problems (uti, pid, etc.).