We've had a sickness bug in the family. DD1 came down with it on Thursday afternoon, followed by me a couple of hours later. She had it really badly - lasted about 8 hours. I had it not so badly - about 4 hours. Both of us were sick and had wierd diarrhoea - very dark and thick.
48 hours later, DD2 (nearly 6) and DD4 (2) got it. They didn't get it nearly so badly - just threw up three or four times over the course of 2 or 3 hours and that was that. DD2 did have one bout of the weird diarrhoea.
DH just felt nauseous for a day and DD3 (nearly 4) seems to have got of scott free.
Now DD2 (the one I'm worried about) does seem to suffer from IBS every now and then - she'll get stomach cramps, and then go and do very watery diarrhoea. She had an episode before she got sick, but I think it was the IBS, not the illness because of the pattern.
THe day after she was ill (Sunday), her energy was like normal, she was eating normally within 12 hours, and she was completely herself.
Yesterday she got very tired in the morning, very unlike herself, but I just put it down to her recovering from the sickness bug. She hardly ate anything yesterday. Then, in the evening, she complained of stomach cramps again, and had more watery diarrhoea. She went straight to sleep when she went to bed, which is unlike her. And this morning she is still in bed at nearly noon. (She's home educated so wouldn't be in school anyway now).
She has been dozing on and off all morning, and, when asked, says she's tired and has tummy ache, but doesn't feel ill. She's not had an episode of diarrhoea since yesterday evening.
She's been drinking ok yesterday - mostly water and ice-cubes - but nothing this morning so far. She said she wanted to get up for breakfast about half an hour ago, but then sank back down on the pillow and I said, 'why not stay here for a bit longer?' and she said 'ok' and closed her eyes again 
So now I'm starting to worry. WWYD?