Help: 31 weeks pregnant, consistent contractions

Female, 36, 31 weeks pregnant, 5'2, current weight 118 lbs, pre pregnancy weight 97 lbs.

Last night, I went to the hospital because i was having some pretty consistent braxton hicks contractions. Painless, but they were coming at about 5-7 minutes apart, lasting 15-60 seconds, in an hour.

They checked the fetal heart rate, did a quick ultrasound to check baby position, and an internal exam to check the cervix. All were good. Heart rate was 148, baby still breech with bum right on top of my cervix, and cervix was still long and closed.

A nurse said that a Uti may trigger braxton hicks contractions so they did a utine test. The results show

elevated WBC (21-50 HPF)

RBC (11-20 HPF)

leukocytes (500 Leu/uL)

Bacteria, urine (>20 HPF)

squamous epithelial cells (>10)

Cloudy urine

No nitrite, protein, ketones, and glucose.

Urine culture was also done but results wont be in until 2-3 days later.

The contractions are still coming and going but i noticed that my belly softens when im laying on my side. Once i sit up, the front middle of the belly start to immediately tighten up. No pain in the back, no cramping in the lower belly. Just an uncomfortable tightening sensation that only goes away if i change positions.

Doctor prescribed anti biotics (amoxicillin trihyrdate) but said if i wanted to wait until i got the utine culture result, I can.

What could these contractions when sitting mean? Also, will the antibiotics have negstives effects to the baby?