It is narrated (marfu'an) from Hazrat Safwan bin Umayyah (may Allah be pleased with him) that dying from the plague, from a stomach illness, by drowning, or in the state of postnatal bleeding is also martyrdom.
Hadith Referenceمسند احمد / من مسند القبائل / 27635
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد ضعيف لجهالة عامر بن مالك