Severe nausea, back ache head ache, cramps, lower stomach pain, sexually active been having for 6 days now on bc may not have taken correctly.?

Not the whole story. In someone who is sexually active and now with nausea, back pain and pelvic pain, the first thing to check is a pregnancy test. If you are pregnant, then you need to seek help immediately. If the test is negative, then the answer is yes, taking pills incorrectly could cause some breakthrough bleeding and cramping.