In 5th day of clomid

I'm 43 yrs old, i had blighted ovum last june, trying again for 3 months, thus my doctor put me in clomid for 5 days, and today is the last one. Dr said wait for 3 days then have sex every other day. I started opk but what if ovulation starts tomorrow can we have sex? Or after 3 days? My cycle is regular until the last 2 months was way too ahead from 26 days to 35 days... is it ok to try also preseed? Is it safe?