I'm sorry to hear that you experience a reaction to the flu shot
. Reactions to the flu shot are rare, but they can occasionally occur, especially in those who have a history of severe allergies to eggs. It is hard to tell whether you had a serious reaction or not based on the symptoms you are describing so it is important that you speak with your OB GYN. For example, if you just broke out in hives
, then this would not be likely to represent a severe reaction, and there would be a very remote chance that this could have affected your pregnancy. If you experienced a more severe reaction, called anaphylaxis, which usually involves trouble with breathing and changes in the blood pressure, then this could theoretically affect the pregnancy by affecting blood flow through the placenta temporarily, especially if it was not treated promptly.
I would recommend that you seek the advice of your OB GYN doctor
at your earliest convenience. Although I think it is statistically unlikely that your reaction to the flu shot would affect your pregnancy, you definitely do need a thorough evaluation. There are also many other potential causes for a threatened abortion, and having all of those causes investigated is quite important.