I'm 7wks & 22yrs old and I don't really know how much I actually weigh because two places told me two different weights. Before I was preggo I weighed 122lbs approximately and when I was weighed at the doctors I was 125 but some other place said I was 121lbs. So I have no idea how much I actually weigh because I don't own a scale. I never really needed one because i've always weighed between 120 and 125lbs. I'm 5ft 7in and I think I am underweight for my height and age.
I had a huge issue with my weight at the start of my pregnancy. I'm 5ft 1in also and weighed 13 1/2 stone before. I started drinking 2-4 litres of water a day as well as keeping track of what I ate and I managed to lose 2-3 stone. I'm now 35 weeks and just hit 13 1/2 stone so I know that while I'm losing weight my baby is gaining. Thinking about your weight too much can not only stress you out but your baby also. As long as your not skipping meals or going without baby should be alright :-)
I was the same way. I just started picking up my weight and I'm 22 weeks. My Dr was concerned a bit but she's just told me to just keep eating. And take your prenatals that also helps. Hopefully you start yo gain some weight before your baby is born
If your doctor isn't concerned about it then you don't need to be either.