Your lender would have had to submit a short sale approval agreement in order for the sale to close. In that agreement, which is formatted much like a letter, would be the terms and conditions which would have been signed by you. A copy of this letter/agreement should have been given to you at closing.
Look over this document to make sure you did not agree to pay the balance or agreed to a promissory note. If your lender was Bank of America or the servicer was Countrywide, it could have been slipped in at the last minute. Also, you should have been previously advised, a home-equity line of credit is considered a non-purchase money loan and can be pursued by the lender. It does not matter if they 1099'd you, they still can try to collect the debt.
They may also file for a deficiency judgment so check your agreement to make sure it clearly states they will not pursue a deficiency judgment. Although they may file, it is unlikely they will not.
What is more likely, is they will continue to pressure you unless you prepare yourself properly. 1) Find that short sale approval letter/agreement to see what grounds they may have for collection. 2) Read it very carefully and look for any wording like "we reserve the right to collect this debt..." or "reserve the right to pursue any and all actions available". 3) If there are any of the above phrases, then you may wish to use an attorney to advise you.
If you have any questions, I have a lot of experience in these matters and can help you a little. You can reach me through my site. I am not an attorney or RE Agent, so I can only give you the benefit of my experience and certainly will not charge you. Good Luck!