This is a classic example of serious symptoms-- You must seek veterinary attention immediately in this case. If the skin is yellowing, that is a sign of build up of bile pigment in the blood stream- this can happen from rapid break down of red blood cells (which means there is likely severe anemia or low red blood cell count). It can also occur from a blockage of bile flow or processing of bile in the liver- either an obstruction of the bile flow out of the liver or a problem with the liver tissue itself, not being able to process bile. All of these situations can be very serious and potentially life threatening. Possible diagnoses that come to mind are Leptospirosis (a bacterial infection that affects the liver and kidneys and can be fatal), severe anemia secondary to tick related diseases or hepatitis. Please see your vet immediately and let us know how things worked out. Wishing you the best for you and your pet!