Is this food poisoning or stomach flu?

I’m sick, and I have no idea what’s going on. Here’s what happened. First, I woke up at about 1 am, Wed morning with terrible stomach discomfort and diarrhea. That lasted for about 3 hours—I slept on and off, poorly. Then, at about 4 am, I woke up feeling awful, promptly vomited, and immediately felt better. I chalked it up to some bad watermelon (the only thing I had eaten that evening, weirdly enough.)

Wednesday I felt weak, but Wednesday night I slept perfectly.

Then, seemingly out of nowhere, I experience Part 2 of this nightmare at 1 am, Friday. No vomiting this time, but plenty of discomfort and diarrhea. Now it’s Friday morning and I feel weak, very dizzy, and just BAD. I’m at work, and I’m afraid I might have to duck out of a meeting to throw up. Ugh.

What do you think? Could I have somehow been exposed twice to whatever gave me the food poisoning the first night (I was very careful and did not eat anything I had eaten the night before)? Or is it not food poisoning at all, but a stomach flu with an odd break in between?

I’m afraid it’s going to keep happening unless I determine the culprit. Help!