When hugging your boyfriend, you might feel a hard lump in his pants, which is commonly referred to as a "hard-On". This is a normal physical response to the physical closeness and touch involved in a hug.
The cause of this hard-on is the physical closeness and contact between your bodies, which can stimulate the male pornographic area and lead to arousal. This is similar to the sensation you get when you become excited during sexual activity. While it may feel strange or尴尬, it is simply a normal physiological response to being affectionately touched by someone you are attracted to.
It's important to note that getting an erection during a hug is a normal and common occurrence, and should not be a cause for concern or shame. It doesn't necessarily indicate anything inappropriate or abnormal, but it may take some time for you to get used to the feeling.
Most people will find the feeling of a hard-ons while hugging to be flattering and pleasant, as it shows that the person enjoys being close and affectionate. If you are concerned about it, it is possible to discuss your concerns with your boyfriend and try to communicate openly about your comfort levels and boundaries.