How do you tell if your dcs have a spring cold or hayfever?

[Overrun]

We are having a tough time in the Overrun household, as the dcs and myself have been ill off and on for months.
They have been snotty for ages, and now have streaming eyes and sneezes, but ds1 also has a high temp and a productive cough. I haven't been able to find out the colour as he doesn't spit it out tmi I know
Also, to make things more confusing, we all seem to have digestive problems. Dts have had funny tummies for ages but seem well in their self. Ds1 is ill with it, having explosive diarrhoea, which unfortunately I have now got.
Anyway, I am giving them piriton, but it doesn't seem to make any difference. Also Capol, god I don't know.
I suppose in a way, no body here is going to be able to give me the definitive answer. Its just frustrating not knowing what is causing what. Its not helped by the fact that ds1's Asthma is quite bad now, and I am beginning to feel I should get him seen by a gp

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Maybe Happybiggirl - ds1 has Asthma and so does dt1, and probably dt2, although he doesn't take any meds atm
I have heard from a variety of sources that hayfever is bad this year, although mine is not too bad. I just don't know whether I should keep giving them piriton if it doesn't seem to have any affect.
I am worried by ds1, he is very pale and asking for his inhaler which is unusual. Might have to take him some where tomorrow