Seriously help me please my baby is sick?

so my daughter is 5 mth and she has a cold, and gradually i noticed its developing into a crackle on her chest so we took her to the doc on the 10 of sep she said she did not hear any crackle but that does not mean she don't have one so she gave us some medication for her. but what worries me badly now it seem to be getting worst she now has a runny nose that occupationally gets blocked and also a nasty cough along with the crackle on her chest. How long does a baby normally takes to get over a cold? cause this has been going on for close to a month