Hi.. Thanks for the query.. Tongue swelling can be due to allergic reaction that can be most probably due to either any allergen in ice cream or the antibiotic Doxycycline.. So my suggestion is to first of all take anti allergic like Benadryl to reduce the swelling and immediately consult an Emergency room as the swelling of tongue can interfere with breathing and swallowing.. You can be advised a steroid shot to reduce the swelling faster. Once the swelling gets reduced, oral antihistamines/ anti allergics can be continued for complete relief.. Also an Allergy test can be done to rule out the cause of the reaction and treatment can be further continued accordingly.. Hope this helps.. Regards.