Women in their last couple months of pregnancy sometimes walk with a waddle. Your body is releasing a hormone called relaxin, which loosens the joints in your pelvis (and everywhere else in your body). Also, the weight of the pregnancy stretches and puts a lot of pressure on the groin area. Those two factors can affect how you walk and are likely causing the ache in your groin.
If your ache turns into pain, or if it becomes unusually difficult to get around, consult your health care provider. Otherwise, wait for it to go away on its own after delivery. But be patient! It may take up to six months for pelvic pain to go away after pregnancy.