What Causes Tightness In Vagina During Intercourse?

Well I recently , started going out with a 54 year old man . We ahve been together for 2 and a half weeks we tried to have intercourse several times . But when it comes to sliding in , I seem to freeze up and my vagina seems to close down. We do have long oral sex before as he finds it difficult to climax quickly . We talked it over and would love to sort it out as quickly as possible . I am 51 years now . And this is the first time that this happened . He also believes that we can sort it out So that is why i am emailing now as we both do not need another heartache .I also use the lubrication liquid Astro glide as ?I am becoming dry and am in menopausal period last period was in N ovember . Thanks in advance for your help .