Uterine bleeding can make a woman niddah even during pregnancy. Whether or not you are in niddah does depend on the source of your bleeding. If you have not already done so, you should have a physical exam. If your doctor finds a non-uterine cause for the bleeding you experienced after your last relations, please get back to us or a local halachic authority with details.
If the bleeding was light (spotting rather than a flow), and you did not see it exiting your body or notice it immediately after relations, then the leniencies of stains apply. Please see our article “Ketamim” for more details and get back to us with any further questions.
Otherwise, you should assume that you are in niddah, in which case you will follow the usual process of becoming tehorah.
Usually, there are some stringencies attached to bleeding immediately after relations, but these are relaxed somewhat during pregnancy. Should you have bleeding immediately after relations without a good medical explanation, please get back to us.
BeSha’ah tovah!