You need to go to the doctor, you may need some physio - all your joints weaken during pregnancy and you must protect them. Do you sleep on a hard bed? If so, you will probably need to get a soft topper. I like a very firm bed and also liked to have a nap on the floor from time to time. When I was pregnant I ended up with sacroiliac hip disfunction because of lying on the hard surfaces! By the time my baby was due, I could barely walk, couldn't carry anything and could only get up stairs by crawling - this also caused problems during delivery as I was very restricted in the positions i could get into and I ended up with an emergency c-section. The problems disapeared within days of the baby being born, and because I knew better , my second pregnancy was a lot better, but I still have hip weakness from this.
So what I am saying is - get it checked out now - pain killers etc will only mask the problem.
I know that you said you had this before you were pregnant, but the pregnancy hormones will make it worse and you need to get some help.
Best of luck
p.s. not sure about the deep heat etc, but I would guess you should not use it - what does it say on the leaflet? What about a warm pad or hot water bottle or leaning your back against the radiator while sitting on a stool or a chair with the back turned to the side.