It seems like collection of the blood around the knee "HEMATOMA".
Based on the information it very important to know what is going on in your knee, but since you are pregnant you can't go for an X-RAY , the other viable option is "ULTRASOUND" & "MRI"- can be used in such cases.
Best thing to do is to consult an ORTHOPEDIC surgeon near you and have him look at your condition, it can happen that based on his experience he may come to diagnosis even with out going for "MRI".
CONSULT ORTHOPEDIC SURGEON
Here is what they normally tend to do
1) Physical exam,
2) May need to "ULTRASOUND" Depending upon the FLUID. and if blood it can be extracted out via needle. / MRI - in case of ligament tear.
3) May give conservative therapy like heat, cold
therapy since you are pregnant.
4) May need to give a local injection to reduce the swelling to get it better quickly in case of swelling.
5) May need to inject pain killer depending upon the pain and situation.
HOPE IT helps.