My husband has an illegitimate child

My husband has an illegitimate child

I have recently found out that my husband has an illegitimate child (now38). This happened when he was living abroad in Canada before I met him. I only found out because the mother of the child wrote to my husband and castigated him for not contacting her and the child. My husband does not want any contact and tore up the letter in front of me. I cannot see what this woman hopes to gain after all this time. The only thing that she has gained is that I feel really unhappy and shocked.
I find it so hard to comprehend that my husband kept this secret from me for 36 years of married life and I feel uneasy that he may have other nasty shocks for me. I am trying to keep a brave face on this and keep up a normal relationship, but all the time I am feeling so stressed inside, finding it hard to sleep and have lost my appetite. My main worry is that this woman will keep on trying to contact my husband again, putting such a strain on our marriage.
Am I right to worry?