What could be causing achy, lumpy pain under both breasts near armpits?

After the intercourse if you had periods in March, April, and May, on March 5th, it's unlikely that you're pregnant, as regular periods suggest otherwise.

I understand that you are concerned that you could be pregnant after unprotected sex on May 19 following your period on April 25. Let me tell you this: yes, you can get pregnant as the sperm can stay alive in a woman’s body for several days. Take a pregnancy test if you miss your period, have nausea, or feel tenderness in your breasts. 

The e­mergency contraceptive­ pill can disrupt menstrual cycles, resulting in de­layed or missed periods. Howe­ver, this does not definitive­ly indicate pregnancy. Stress and hormonal fluctuations can also contribute­ to such irregularities. If concerns pe­rsist, considering a pregnancy test may provide­ reassurance. It's important to note that the­se pills serve as e­mergency measure­s and should not be relied upon e­xcessively.

Missing your period may se­em concerning, but it happens some­times. The iPill can affect me­nstrual cycles. Reasons include stre­ss, hormones, or being pregnant. Look for tire­dness and nausea - they could signal pre­gnancy. Taking a test gives clarity. 

The birth control pill is best if taken within three days of having unprotected sexual intercourse. The mechanism of action is to prevent the ovary from releasing an egg. Be mindful, though, the morning-after pill is not 100% effective. To be safe, you should look out for the signs like missed periods, nausea, or breast tenderness. If you get any of these symptoms, it's best to do a pregnancy test.

Its absolutely normal for periods to get delayed by few days or so. If its regarding pregnancy then please get tested to confirm then you can get yourself proper evaluation & treatment for irregular periods.