Back pain it started about 5 days agoe first a little pain when I was carying an esky, was a bit sore the next day by the following night I was of to the doctor as the pain was unbearable. Doctor said he thinks it's pulled muscle but it's still really pa

Did you have pulled. Muscle, you could your self if you have it, pulled muscle should improve with treatment and rest. Some time we could order MRI test to see this a pulled muscle if did continue more than usuall time.
He may be right. Start an anti inflammatory medication ( i assume he prescribed one) like Ibuprofen for a few days, gentle stretching, ice packs and if not improving over the next week or two, follow up with him. Most back pain resolves with conservative treatment. If its not improving then more advanced imaging may be needed to look for other causes.