Thanks for posting your query.
In third trimester baby gains maximum weight & if a mature fetus is moving inside it can cause pain by pressing surrounding viscera.
Light contractions in pregnancy
are normal known as Braxton Hics contractions & when there are converted to painful contractions, then patient is said to be in labour.
But your swollen vagina is a matter of worry.
For that consult your gynaecologist as soon as possible & get your blood pressure checked & urine albumin