Painful eye, sensitive to light and watering. slightly bloodshot and red round the iris. had a similar thing last year was told it may be allergy?

Likely iritis: I would say you have recurrent inflammation of the anterior segment called Iritis (Uveitis), likely related to your systemic condition of low back pain. If you know you have ankylosing spondylitis (inflammatory arthritis of the low back), then this is associated w/ it. You need a complete ophthalmological exam by an Eye MD, who will evaluate & treat. With a recurrent history, you need a work up.