Another thing that may be relevant - being 18 you're going through an extremely horny period and probably getting turned on at various points during the day, from different stimulii. You may well be pre-cumming a lot, some guys do (I'm one). Your underwear will soak up any "spillage" but there will obviously be some retained under the foreskin. Just make sure that when you go for a pee you pull back your foreskin and dry under it when you've finished and you may also find that you need to give your cock a wash at a convenient point once or twice during the day, especially when it's been hot and you've been getting turned on.