Phlem is essentially the dead white blood cells that fought your respiratory infection. They tend to congregate where the infection is the lungs. If you were coughing hard its absolutely normal that you would have broken blood vessels and had a small amount of bleeding in the lungs and phlem would gather at the sight of that damage as white blood cells fight the infection and die, clotting the wound. As the phlem is coughed free it takes the blood that was absorbed in the phlem and sometimes it dislodges wet scabs from the walls of your lungs that can cause a few drops of blood in the phlem. This is a sign of serious pulmonary infection but also actually a sign that your body is healing well. Any time you have thickened phlem or small amounts of blood in the phlem it's a sign that you need to take care of yourself, sleep well and keep drinking fluids to help support your immune system.
If you're seeing more than just a drop of blood in your phlem or if you're seeing fresh red blood consistently for more than a day there could be other complications from the infection and it's best to have it checked out by a doctor.