Help! What is the yellow stuff on my stitch? (Photo)

It looks like an infection to me. You should seek medical attention right away from a surgeon who does this procedure regularly.  Be sure to choose a board certified plastic surgeon that specializes in this procedure and is well known in the medical community for it.  

Dear Nazasprob,   Thanks for posting your picture, though , fully exposed butt would have been helpful.    From observing your pictures, you are justified to be concerned, because , I see spreading redness around the wound, which could be cellulitis (infection). The yellow stuff on the wound is most likely Fibrin.     If you were my patient, I would have sent fluid specimen to the lab for bacterial culture and sensitivity. Additionally, I would have treated you with antibiotic like Bactrim DS, in case of infection by MRSA (staph resistant) and instruct you in wound care.   All my butt implants wear tight Velcro binder 24/7 for one month after surgery, to protect the wound and reduce the likelihood of bleeding.   These are important measures , because once the implant gets infected , it has to be removed.                   Best of luck,                                     Dr Widder