Hi and welcome to the forum! I am glad that I can help you.
You want to know the cause of the symptoms that you experience.
During the procedure of getting a tattoo the skin is injured and there is a possibility for bacteria to get in the skin and cause infection. Skin infections are characterized with redness of the skin, visible or invisible edema, pain, elevation of the skin and pus or discharge from the change on the skin. Individuals, especially those who are prone to allergies, may have an allergic reaction on the colors used in tattoos. An allergic reaction on the skin is characterized by redness of the skin, itching and elevation of the skin (small or bigger).
Given the data that you decribed, you may experience one of the conditions listed above. See your doctor for further diagnosis. You may need to take antibiotics if you are diagnosed with a skin infection. Avoid body lotions or butter to avoid the spread of possible skin infection.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please contact your doctor for further advice and information about diagnosis and treatment options.