Dry caramel 'curdles' when adding hot heavy cream
When havy cream is added after caramelizing the sugar, it's done to stop the caramelization process, keeping the desired caramel flavor and preventing the caramel from becoming bitter or even burning (if making caramel for sweets/pastry coating, the caramel is used as is and is cooled by placing the pot/saucepan in a cold water bath).

When adding cream to the caramel, using cold cream will rapidly cool your mixture, keeping the caramel at a temperature more suitable for a sauce, whereas using warm cream will keep the caramel closer to a softball temperature more suitable if you're making soft/chewy caramels, which could cause the edges of the caramel to thicken and start pulling together.

Since you brought the cream the a boil, your caramel probably hit the softball stage while you were stirring the two together. Try warming the cream to a lower temperature; the cooler the cream is, the more your mixture will bubble, but cooler cream will allow for a smoother caramel sauce and a larger margin for error to stay away from the softball stage.

Once you've hit the softball stage, your caramel will not be able to be stirred back into a sauce, you'll just have really soft chewy caramels, or you can heat them longer for harder chewy caramels (I highly recommend a candy thermometer at this point, or if you're particular about the density/chewiness of your soft caramels).