Why is there pressure in my lower abdomen? is it a constipation sign or something else?

Depends: This is a symptom. It depends on whether there are other syjptoms---evidence of bowel dysfunction etc. If it is a solitary symptom it may just indicate mental tension. See your physician.

Too vague: The question is too general. Without additional information and a physical exam it is impossible to say. If you are concerned a visit to a physician may be indicated.