Constant cramps and brown spotting 8 weeks

So I have had a pretty stressful pregnancy thus far. I am 8 w 3 d. I have been constantly cramping and brown spotting on and off for weeks. I have been seen by the ER and my OB on several occasions. I had an u/s yesterday and my baby was measuring perfectly and had a HR of 172. I am taking magnesium per OB and staying plenty hydrated. I am just stressing out constantly. Have any of you experienced this? I am terrified of losing my baby. Thank you! This was my u/s from yesterday.