There are a number of contra-actions (side effects), that may be experienced by the client after sports massage. Whilst they are a normal physiological response, and generally mild, they can be concerning for the client if they are not made aware of these. Whilst these are usually temporary and mild, the most common contra-indications include: 

Soreness: It's common to feel some muscle soreness for a day or two after a sports/deep tissue massage, similar to the sensation of post-workout muscle soreness (DOMS). This is usually a sign that the massage has effectively released muscle tension. In addition, may result in soreness or dysfunction in other areas as the treated area may have previously restricted the range of movement at other joints.

Bruising: Occasionally, during the first 24 hours, you might experience minor bruising at the site of deeper pressure or on sensitive areas due to the trauma inflicted on the area and increased vasodilation. 

Fatigue: Some people may feel temporarily fatigued. It's essential to rest and hydrate after the session.

Headache: some people may experience a mild headache, usually due to the release of tension in neck and shoulder muscles, which should subside within a day, or from dehydration.

Flu-Like Symptoms: A very small percentage of individuals may experience flu-like symptoms including headaches, runny nose and nausea due to the stimulation of the circulation and lymphatic system. This promotes the sudden elimination of waste products and toxins, which can also trigger an immune response resulting in similar symptoms that would be experienced during a bacterial or viral infection. If experienced, it should only last up to a day.

Emotional Release: Deep tissue massage can sometimes bring up emotional responses, such as crying or laughter. This is a normal and therapeutic reaction for some people.

Intense Relaxation: Deep relaxation during a massage can sometimes lead to a temporary drop in blood pressure and heart rate, potentially causing feelings of light-headedness or weakness.

Increased urination: An increase in blood and lymphatic circulation can stimulate the kidneys to eliminate metabolic waste and toxins from the body.

Change in sleep patterns.

Dehydration: The massage can stimulate the lymphatic system and promote the release of toxins, which can increase the body's need for hydration. If you become dehydrated during or after a massage, it can result in symptoms like headaches, dizziness, and fatigue, which might be mistaken for flu-like symptoms.

Low Blood Sugar. It's a good idea to have a light meal or snack before your massage if you're prone to low blood sugar. If you are diabetic, you should test your sugar before treatment.

In addition to the above, the following techniques may have some specific contra-actions 

Cupping:

The skin in and around the rim of the glass cup can be red, this can last anything from 5 to 14 days. This is normal. Cupping marks rarely hurt but can sometimes be tender if the rim of the cup as had pressure over a bone. There are 2 schools of thought regarding the marks left. Traditional healers will tell you that the marks left are caused by the drawing up of old non-circulating stagnant blood, sticky fluid and toxins from the deep layers of the tissue to the surface of the skin. More science driven therapists will tell you that the marks are left to an increase in blood vessel density, ruptured capilliaries.

On rare occasions, a blister or blisters containing fluid may appear inside the perimeter of a stationary cup. Caution needs to be taken to keep the area free from infection. The blisters should be carefully wiped with antiseptic, then covered with a sterile pad and left to reabsorb into the body.

IASTM:

The treated skin may be sore, bruised or exhibit small red dots over the area treated, called petechiae (where scar tissue and adhesions break apart, thus, breaking capillaries near the skin’s surface). 

In the event that severe pain or discomfort is experienced, they should be advised to seek medical attention.