Are you having intercourse with the same partner? It could be as simple as you aren't as "wet", lubricated as normal and you tore a little bit which caused you to bleed. That can happen and it's nothing to worry about. You can try using KY jelly and see if you still bleed. If the bleeding continues, I would go in and see if there is anything wrong.
Usually spotting can also occur if you have rough sex...or have extended intercourse through an evening. All of these things will cause a slight bleed because the tissue is so tender.
If it continues, get in to your GYN and find out what's going on. 'k?