See. Look. Daisy. Not trying to be all self-centred here, but here's a typical recent Lupus post, in between debating the finer points of the problem of theodicy and whether people working at home can think with their shoes off:
Hi
I feel for you as we are just coming out (hopeful emoticon) of another 3-4 month period of constant illness in our 3 kids, though it is our middle child who causes the most worry. We finally insisted on a hospital check up when it got too ridiculous and they did a full blood test which turned up nothing. They don't like doing it on little kids but it put our minds at rest. They did suggest he had asthma and has been on steroid inhalers since. Not relevant to your situation but the underlying condition being more manageable has perhaps led to fewer chesty coughs and tiredness and therefore less general unwellness.
So I second others' views about getting the diarrhoea fully checked out again. Go back to the GP and ask for a full review of what it could be a symptom of? Ask to be referred on?
But should also say, it has been like this with all my kids in the first year/18m of nursery. They have to get these things to build up an immune system (and kids immune systems don't work very well until they're a bit older) and some kids - yours and mine - get things relentlessly.
FWIW DS1, now 10 was off seemingly every other week, and now has the consitution of a horse.
I'd check out the bowel thing again, get some extra hours of help to cover the work things, and a bit of moral support for you. It is really, really hard and draining. I speak from experience - have nearly gone nuts 3 times this year since returning from Mat Leave. You've gotta throw your hands up and give in and sleep when you can, but so hard to manage with work. Is your DH back soon? Can he be? Does he understand how difficult it is for you to keep all these balls in the air on your own?
Hope you get a break soon