My 17 mo old is terrified, WHY?

My daugter is a people person. She waves at everyone at the store and says hi to everyone. She is 17 months old. She is very affecionate and soical. Lately for the last two months when people come to the house or knock on the door even if its a light tap pn the door she freaks out and cries like someone is beating her or killing her! She runs behind me or behind the table or falls to the ground and acts EXTREMLY TERRIFIED! It is very annoying because i can tell it offends everyone who comes here including my nurse because im 7 mo pregnant and i have daily nurse visits.After the person comes in sits down it takes my daughter almost 20 mins to calm down and stop crying and screaming at the top of her lungs! They always ask whats wrong with her. My friend recently came over and this girl comes over every 2 days and has been since my daughter was born. My daughter freaked out when she walked through the door and ran behind me and when i ignored her and kept walking she went CRAZY and ran into the entertainment system and now she has a big bruise under her eye, which makes it worse because now when people come they probably think im beating her! I dont undertsnad because she is friendly to everyone at stores and church and the mall but when someone comes to the house she FREAKS OUT!! I have even tried telling my friends to call in advance so I can open the door so they can walk in and that doesnt even help, she still acts maniac!!