Erection issue some time. For 1month!

Planning for family addition consulting doc told to have intercourse on Sunday been fertile period, I did in afternoon with full erection and good quantity but when tried doing in night erection was not firm and also tired so slept next day on Monday doc told to do intercourse for next two days also so in evening tried to intercourse again but again the erection not happen then I got tense tried for few min but slept without any success as doc told to do I tried to do on today morning but erection happen after lots of effort by shaking the penis but when i had intercourse was not able to last long So i am bit worried what happen to me all of the sudden my sex desire became zero even did sex last Tuesday and Sunday properly with firm erection Pl make me understand this is normal or it's age or ed problem doc recommended parternia xt had for 1month