You ask a very good question. Getting any part of your body pierced carries a certain amount of risk, and it is great that you are doing some research ahead of time. Other FX readers have had similar questions, so you should definitely check out Considering Nipple Piercing for a discussion of some of the risks of body piercing.
In general, the most common risks of getting any body part pierced include infection and disease transmission. If the piercer does not use sterile tools, you could be at risk for getting HIV, hepatitis B or tetanus, all of which can lead to fatal illnesses. In addition, bacterial infections are fairly common; especially weeks after the piercing is done, and can cause pain, swelling, and scar tissue formation. This can lead to possible disfigurement of your body, which may or may not be able to be fixed with surgery. Your body can also reject the piercing, and can push it out from the surface of your skin, which may lead to scarring, too.
Specifically with nipple piercing, FX has not found any association between weight lifting and nipple sagging or stretching. However, some sources say that a lot of people do experience nipple enlargement after piercing, which does not often go away after the nipple ring is taken out. Also, some people may get numb nipples from the procedure, which is a permanent consequence of the piercing. Some people also complain of their nipple rings getting caught on things like their clothes, which can cause trauma to the nipple and may lead to infection, bleeding, pain and possible tearing of the nipple.
It is possible that your eyebrow piercing could hit a nerve (not a vein) that controls the muscles of your face, which could cause your face to sag. This is more of a risk if the piercer places the ring too deep and very far to the edge of the face or the piercing is done close to the bridge of the nose. While these risks are not all that likely, it is important to be aware of them ahead of time. There is also a muscle that runs around your eyebrow that could be damaged or cut through with a deep piercing, which could lead to problems as well.
FX does its best to keep its readers anonymous, so we cannot send an email to your personal account. But, hopefully this information will help you make a good decision about getting a piercing.