What worming medication are you using did you follow the instructions / dosage as prescribed?
For pups I use milbermax from the vets and it was either once a week or once a fortnight until 12 wks (can't quite remember) but from 12 wks to 6 mths it was once a month. Then 2 - 3 times per year after that.
Some wormers do seem to give them diarrhoea 24-48hrs after taking but I never experienced that with Milbermax.
I personally would ring the vet mainly because of his tender age, tell them what you've given and ask for their opinions on repeating a dose because worms were present. If you don't decide to ring the vet however I wouldn't personally repeat dose knowing more bank holidays are coming up just to air on the side of caution in case he becomes sick from the extra worming medication.