Hello! I had an ultrasound at week 17 because my belly hurt and the so…

Hello! I had an ultrasound at week 17 because my belly hurt and the sonographer told me that the placenta was thickening 33 mm. He said we'd check again in a month. I'm currently at week 20 and have an appointment with my hematologist. My hematologist says I should get another ultrasound to see what's going on. I'm being treated by a hematologist for thrombophilia. Can I get ultrasounds multiple times? Is it dangerous? Nobody explained to me what "thickening of the placenta" means. Has anyone experienced this?