Usually in your first few weeks of pregnancy you do feel period like cramps that are often confused with just that. You might think you are getting your period but it never really comes, uless you are having implantation bleeding.
If you think you might be pregnant do not use tampons, this can cause infection!!
Implantation bleeding is normally light bleeding or spotting. The blood can be a dark brown-ish color or a pink-ish color. The time that this should last depends on each individual. Some women might just spot once one day and some women might spot for a few days.
If you think you are pregnant take a pregnancy test a few days after you missed your period.