J.F.
Been there, done this! :)
While you don't have to give up running all together, you do need to cut down. Especially intensity! I also took up walking which pissed me off to be honest, but I got used to it and it got me out of the house. I felt walking was just not worth my time, but was shocked to find out it still was good for me and kept my body moving....
Also (and this is KEY) you have to gain weight. Not much, but you do have to gain. Also it is about % of body fat. Yours must be too low! You don't have to mac out on ice cream and bad fats, but you should increase good fats... nuts, good oils, dairy ect...
At first I found this all very hard, but it totally works!!! And to be honest I think in the end it made me a healthier person. Talking to a dietitian really helps to know what foods can help you out.
Once I got prego I didn't run much at all... I couldn't keep my heart rate down, so I kept up the walking. Did lots of 5k's where it is ok to be with others walking and loved it. I even did 3 the 3 weekends before I had my daughter and my labor and delivery was a breeze. Everyone I know says it was due to the walking.
Anyhow good luck and just use common sense... you will figure it all out.