Lifting heavy objects isn't going to physically harm your baby or anything. The restrictions for heavy lifting are in place because of how the woman's body changes to accommodate for bub meaning that all the muscles and ligaments have separated and relaxed to make room. Which lifting heavy objects can in turn, you can hurt yourself by doing so. I'm 25 weeks and lift heavy objects all the time, have done the whole pregnancy. Just know your limits and all should be fine and dandy. Your body will let you know when its time to stop and rest. So just relax for a few days and if it gets worse or you become worried, contact your doctor for advice :)
Hi, I'm 23 weeks and I have a 9 month which in a few days he'll be 10 and I know how you feel. I've felt like that yesterday and tried to lay down as much as I could. Lifting your son up, he needs to be taken care of. If you can get some help, would be much better. But lifting heavy objects, you shouldn't. If you still feel like this and more constant, I would call OB.
Good luck!