Diarrhea within half an hour every time I eat.

The only obvious answer is to go a hospital if you have travel and medical insurance. Failing that a pharmacy.

You have obviously contracted a bug..perhaps from the clams not cooked enough.

Best wishes...

You know there are two very different types of food/water poisoning and first of all: food poisoning can last weeks or months.

The one is, when you pick up an active 'bug' (mostly due to undercooked food or contaminated water), some bacteria or virus or parasite, that is pathogenic for you and lives in you, plays havoc, multiplies, leaves toxins....until tackled by immune system or medication.

The other one is, when you eat a chemical, a passive (non multiplying, not living) toxin, that was either made by a bacteria (and bacteria even killed through cooking - e.g. cereus bacillus form toxin at room temperature-stored rice, staphylococcus toxin when people with pus infection handle food) or decaying fish/scromboid or

actually a natural toxin eaten by otherwise wonderfully fresh seafood poisons you (Ciguatera toxin, dinoflagellate product eaten by fish and shellfish mainly in warm water oceans. or venerupin-very bad one, you are not fitting in with symptoms, relief. and many other dinoflagellate products that act as mostly short term food poisoning in seafood) - etc. Rarely food poisoning is man made: pesticides

The list of possibilities is awfully long, very different in its nature and actions and fills books!

You should have had some  soju with your food. ;-)

No, seriously:

Now  your onset was very instant -10min?-, if the clams in pasta were the reason (who knows, maybe you picked somthing up 12 hours, days beforehand and broke out just 10 min after eating this pasta by coincidence...??).

Any bacterial, viral infection would not be that instant.

A campylobacter needs 2-5 days to give you symptoms (awful symptoms btw)

Even listeria, e.coli, salmonella, norovirus need many hours, if not a day at least to develop symptoms since they need to multiply and settle in your guts.

Either it was not the clams and active bug infection or it was a passive toxin with instant reaction.

The reasons are so many, it is impossible to tell.

Unless your stool or blood is tested. And then again, some are overlooked or not tested for like cryptosporidium, unless specifically asked for. (crypto takes btw 2 weeks to heal)

What you need to do is to keep yourself very well hydrated -which you are doing- with not contaminated water, get also so electrolytes into you (but don't overdose!)

An otherwise healthy adult has great compensation mechanisms in place and can cope with diarrhea very well, even for weeks, but you need to supply your body with that what it looses on a greater scale.

Do stay away from fish and shellfish currently please.

Eat anything well cooked and freshly cooked, still of course if you can't cook for yourself, it's always a trust thingy since you don't want to put another infection on top of it or boost it.

If you have a travel insurance and feel getting worse and weaker, you need to seek medical advice and tests. 

If you feel you can handle it by compensation conservatively (hydration/easy digest safe food), then you can wait a little, but be prepared, food poisoning can last long depending what you got into your body.

Food poisoning is not over always in just a day or two, can be, but...it depends what the cause is out of 1000 possibilities for one word: food poisoning.

Best of luck and that you feel better with each day!