I have a sore throat with an ulcer at the back of it, is this strep throat or tonsillitis?

Viral: Throat ulcers are caused by viruses. Strep can have many different appearances (red soft palate, mucus on tonsils, etc) but not ulcers. However if temp stays over 101 for more than 3 days, or if having other worrisome symptoms, get checked by your healthcare provider.

Sore throat: There's no way to tell unless you have a rapid strep test or throat culture (usually not needed). If symptoms resolve on their own in 7-10 days, it is almost always a viral infection. If the ulcer increases in size, more ulcers form, or if you develop fever of worsening after 10 days, a doctor should check this.

It might be!: Have your throat looked at by a trained health care provider when you see something like an ulcer and have a sore throat. Strep typically causes an exudate or white spots, so it depends on what the "ulcer" looks like. A strep test, possibly a Rapid Strep can help. If you still have your tonsils then they are often inflamed with a viral or bacterial (strep) sore throat.Inflamed tonsils=tonsillitis