19 weeks pregnant and High BP and no weight gain for last 3 weeks?

I am 19 years old. I am 19 weeks pregnant . I had gained 2.5 kgs last month (my 4th month) and my BP was normal , I had skin itching and went to doctor yestereday she checked my BP it was 140/80 and she told it was high and asked for symptoms of Preeclampsia which i had none , she told it might be due to stress and asked me to relax and control my intake of salt.She also checked my weight and i had not gained anything, she told me that i should have gained weight if i don't gain weight then it will harm my baby. I have been eating alright.We asked weather ultrasound scan is required to be sure , doctor told she will do ultrasound in next visit which 2 weeks later. Why I have not gained weight and why my BP is high?Is my baby alright? This is my first pregnancy , please help me.Me and my husband are very much worried.