The condition you described is based on a physical reality, a tightening of the vaginal muscles, even though she consciously wants penetration and enjoys foreplay. Why this has happened after five months of marriage is impossible for me to say, without having any counseling sessions with her, and without seeing a gynecological report.
By classical medical definition she has vaginismus (DSM-IV TR diagnosis 306.51 - Vaginismus). It is treatable. I am sure it brings a great deal of anxiety
to both of you. Rest assured, it can be fixed and if you follow treatment, whether professional or self-help treatment using authoritative information.
Having the will to have sex is a very positive sign. Nevertheless, the strong contractions are real. This condition is fairly common, and is just about always overcome.
Have her see her gynecologist first, just to rule out any other causes of this muscle spasm. I also recommend seeing a licensed counseor or marriage and family therapist who is ALSO certified as a Sex Therapist. This is their area of expertise and their profession involves this kind of problem.
Your marriage will be fine. Be positive, be patient, and take action as soon as possible. Everything will turn out fine.
Elliott Sewell, LPCC, NCC