Annpan - I used to work for environmental health and did a food hygiene course so just to put your mind at rest a bit:
An almost immediate vomiting reaction to food is usually because of an intollerence to the particular food - my Mum once made a quick shepherds pie with tinned mince then took us to the fun fair. Before we even went on any of the rides or ate anything there, I threw up right outside a burger van!!! Not the best advert in the world and I heven't eaten it since!!
It could also be because it has been directly contaminated by something e.g. cleaning products etc (no accusations - but if someone has been squirting airfreshener while food is being prepared that could do it!). Also dairy products tend to absorb chemicals stored next to them.
If it really was food poisoning then that takes around 36 hours after the dodgy food has been eaten to kick in. After that - its pretty much as Chickpea says - sniff it, taste it and if it seems ok then it probably is! If in doubt - throw it out!!
HOWEVER.........
Be very careful with things like cooked rice and lentils etc - the bacteria multiply very quickly in these and you can have a direct reaction to the toxins they produce. So if you have leftovers of these its best to refrigerate and eat the next day but NO later or better still freeze them!
Hope thats of help and happy munching!!!
As to the butter - we've found that a 2 litre flip top kilner/luminarc jar is just the job. Just make sure it stays cool whilst shaking or chill in between (the cream and you!! lol)
Regards
Helen and Chris