Preventing a chemical pregnancy...please help, i'm so worried!!!!?

i am on my fourth round of clomid. now last month i either ovulated twice or ovulated very late in my cycle. i ovulated on day 26 and implanted on day 33 (i know this because i had bleeding). i ended up having a chemical pregnancy five days after implantation. i believe it may be because i implanted to close to my expected period. i have been having 35-40 day cycles. my doctor did another 21 day progesterone test this month but i can't get in to see him until Sept 21st. I think i may have implanted again this month on day 33 (last night) because i had very bad cramping which went away by this morning. i guess my question is, do i have a high chance of a chemical pregnancy again this late in my cycle??? if so, is there any vitamins or natural treatments i can take to try and better my chances? i have endometriosis too if that helps....please offer any suggestions you may have. i have to wait to take a test since i only got a very very faint BFP last month four days after my implantation.