How long does it take for symptoms of pregnancy to kick in a

My gf and I had sex before I went off on holiday, now that I am back, she wanted sex, but she was complaining about pains and a late period as well as tender breasts, she has also bled just a little, just being drops and this being 3 weeks after we had last had sex... She's also under a considerable amount of stress... All the symptoms point towards pregnancy, but is it normal to have symptoms 3 weeks after??? Or could the symptoms be due to the stress?