Hi Willow,
I'm a student naturopath and qualified pharmacist so I think I can answer your question: Yes it's safe to take larger quantities of Vit C in pregnancy: your body can only absorb a certain amont at one time (about 500mg-1000mg) so it's best to space out your doses eg one tablet 3-4 times a day rather than 4 tablets at once. You can also increase you intake of zinc fOr a while if you feel a cold coming on (you probably have some zinc in your pregnancy-multi vitamin if you're taking one - add one extra zinc only tablet daily for a few days). Also echinacea is a safe immune-booster in pregnancy (the liquid form works best - it will 'tingle' on your tongue if its a good one).
Finally REST is the most effective medicine for an on-coming cold so take a warm bath/shower, get your partner to bring you hot cups of tea/lemon & honey and spend today on the couch with your favourite book/movie